xref: /aosp_15_r20/external/pcre/doc/pcre2.txt (revision 22dc650d8ae982c6770746019a6f94af92b0f024)

-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------



PCRE2(3)                   Library Functions Manual                   PCRE2(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


INTRODUCTION

       PCRE2 is the name used for a revised API for the PCRE library, which is
       a  set  of  functions,  written in C, that implement regular expression
       pattern matching using the same syntax and semantics as Perl, with just
       a few differences. After nearly two decades,  the  limitations  of  the
       original  API  were  making development increasingly difficult. The new
       API is more extensible, and it was simplified by abolishing  the  sepa-
       rate  "study" optimizing function; in PCRE2, patterns are automatically
       optimized where possible. Since forking from PCRE1, the code  has  been
       extensively  refactored and new features introduced. The old library is
       now obsolete and is no longer maintained.

       As well as Perl-style regular expression patterns, some  features  that
       appeared  in  Python and the original PCRE before they appeared in Perl
       are available using the Python syntax. There is also some  support  for
       one  or  two .NET and Oniguruma syntax items, and there are options for
       requesting  some  minor  changes  that  give  better  ECMAScript   (aka
       JavaScript) compatibility.

       The  source code for PCRE2 can be compiled to support strings of 8-bit,
       16-bit, or 32-bit code units, which means that up to three separate li-
       braries may be installed, one for each code unit size. The size of code
       unit is not related to the bit size of the underlying  hardware.  In  a
       64-bit  environment that also supports 32-bit applications, versions of
       PCRE2 that are compiled in both 64-bit and 32-bit modes may be needed.

       The original work to extend PCRE to 16-bit and 32-bit  code  units  was
       done by Zoltan Herczeg and Christian Persch, respectively. In all three
       cases,  strings  can  be  interpreted  either as one character per code
       unit, or as UTF-encoded Unicode, with support for Unicode general cate-
       gory properties. Unicode support is optional at build time (but is  the
       default). However, processing strings as UTF code units must be enabled
       explicitly at run time. The version of Unicode in use can be discovered
       by running

         pcre2test -C

       The  three  libraries  contain  identical sets of functions, with names
       ending in _8,  _16,  or  _32,  respectively  (for  example,  pcre2_com-
       pile_8()).  However,  by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
       32, a program that uses just one code unit width can be  written  using
       generic names such as pcre2_compile(), and the documentation is written
       assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an
       alternative  function that matches the same compiled patterns in a dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.  For a discussion of the two matching algorithms,  see  the
       pcre2matching page.

       Details  of  exactly which Perl regular expression features are and are
       not supported by  PCRE2  are  given  in  separate  documents.  See  the
       pcre2pattern  and  pcre2compat  pages. There is a syntax summary in the
       pcre2syntax page.

       Some features of PCRE2 can be included, excluded, or changed  when  the
       library  is  built. The pcre2_config() function makes it possible for a
       client to discover which features are  available.  The  features  them-
       selves are described in the pcre2build page. Documentation about build-
       ing  PCRE2 for various operating systems can be found in the README and
       NON-AUTOTOOLS_BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre2", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external symbols are exported when a shared library is  built,  and  in
       these cases the undocumented symbols are not exported.


SECURITY CONSIDERATIONS

       If  you  are using PCRE2 in a non-UTF application that permits users to
       supply arbitrary patterns for compilation, you should  be  aware  of  a
       feature that allows users to turn on UTF support from within a pattern.
       For  example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
       mode, which interprets patterns and subjects as strings of  UTF-8  code
       units instead of individual 8-bit characters. This causes both the pat-
       tern  and  any data against which it is matched to be checked for UTF-8
       validity. If the data string is very long, such a check might use  suf-
       ficiently  many  resources as to cause your application to lose perfor-
       mance.

       One way of guarding against this possibility is to use  the  pcre2_pat-
       tern_info()  function  to  check  the  compiled  pattern's  options for
       PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF  option  when
       calling  pcre2_compile().  This causes a compile time error if the pat-
       tern contains a UTF-setting sequence.

       The use of Unicode properties for character types such as \d  can  also
       be  enabled  from within the pattern, by specifying "(*UCP)". This fea-
       ture can be disallowed by setting the PCRE2_NEVER_UCP option.

       If your application is one that supports UTF, be  aware  that  validity
       checking  can  take time. If the same data string is to be matched many
       times, you can use the PCRE2_NO_UTF_CHECK option  for  the  second  and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
       to  problems,  because  it  may leave the current matching point in the
       middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C  op-
       tion can be used by an application to lock out the use of \C, causing a
       compile-time  error  if it is encountered. It is also possible to build
       PCRE2 with the use of \C permanently disabled.

       Another way that performance can be hit is by running  a  pattern  that
       has  a  very  large search tree against a string that will never match.
       Nested unlimited repeats in a pattern are a common example. PCRE2  pro-
       vides  some  protection  against  this: see the pcre2_set_match_limit()
       function in the pcre2api page.  There  is  a  similar  function  called
       pcre2_set_depth_limit() that can be used to restrict the amount of mem-
       ory that is used.


USER DOCUMENTATION

       The  user  documentation for PCRE2 comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain  text  format,  the  descriptions  of  the  pcre2grep  and
       pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
       respectively.  The remaining sections, except for the pcre2demo section
       (which is a program listing), and the short pages for individual  func-
       tions,  are  concatenated in pcre2.txt, for ease of searching. The sec-
       tions are as follows:

         pcre2              this document
         pcre2-config       show PCRE2 installation configuration information
         pcre2api           details of PCRE2's native C API
         pcre2build         building PCRE2
         pcre2callout       details of the pattern callout feature
         pcre2compat        discussion of Perl compatibility
         pcre2convert       details of pattern conversion functions
         pcre2demo          a demonstration C program that uses PCRE2
         pcre2grep          description of the pcre2grep command (8-bit only)
         pcre2jit           discussion of just-in-time optimization support
         pcre2limits        details of size and other limits
         pcre2matching      discussion of the two matching algorithms
         pcre2partial       details of the partial matching facility
         pcre2pattern       syntax and semantics of supported regular
                              expression patterns
         pcre2perform       discussion of performance issues
         pcre2posix         the POSIX-compatible C API for the 8-bit library
         pcre2sample        discussion of the pcre2demo program
         pcre2serialize     details of pattern serialization
         pcre2syntax        quick syntax reference
         pcre2test          description of the pcre2test command
         pcre2unicode       discussion of Unicode and UTF support

       In the "man" and HTML formats, there is also a short page  for  each  C
       library function, listing its arguments and results.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

       Putting  an  actual email address here is a spam magnet. If you want to
       email me, use my two names separated by a dot at gmail.com.


REVISION

       Last updated: 27 August 2021
       Copyright (c) 1997-2021 University of Cambridge.


PCRE2 10.38                     27 August 2021                        PCRE2(3)
------------------------------------------------------------------------------



PCRE2API(3)                Library Functions Manual                PCRE2API(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

       #include <pcre2.h>

       PCRE2  is  a  new API for PCRE, starting at release 10.0. This document
       contains a description of all its native functions. See the pcre2 docu-
       ment for an overview of all the PCRE2 documentation.


PCRE2 NATIVE API BASIC FUNCTIONS

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       void pcre2_match_data_free(pcre2_match_data *match_data);


PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
         pcre2_match_data *match_data);

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);


PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       void pcre2_general_context_free(pcre2_general_context *gcontext);


PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const uint8_t *tables);

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       int pcre2_set_max_pattern_compiled_length(
         pcre2_compile_context *ccontext, PCRE2_SIZE value);

       int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext,
         uint32_t value);

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);


PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);


PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);


PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacementz,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);


PCRE2 NATIVE API JIT FUNCTIONS

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize,
         size_t maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);


PCRE2 NATIVE API SERIALIZATION FUNCTIONS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);


PCRE2 NATIVE API AUXILIARY FUNCTIONS

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
         const uint8_t *tables);

       int pcre2_pattern_info(const pcre2_code *code, uint32_t what,
         void *where);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       int pcre2_config(uint32_t what, void *where);


PCRE2 NATIVE API OBSOLETE FUNCTIONS

       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_recursion_memory_management(
         pcre2_match_context *mcontext,
         void *(*private_malloc)(size_t, void *),
         void (*private_free)(void *, void *), void *memory_data);

       These functions became obsolete at release 10.30 and are retained  only
       for  backward  compatibility.  They should not be used in new code. The
       first is replaced by pcre2_set_depth_limit(); the second is  no  longer
       needed and has no effect (it always returns zero).


PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS

       pcre2_convert_context *pcre2_convert_context_create(
         pcre2_general_context *gcontext);

       pcre2_convert_context *pcre2_convert_context_copy(
         pcre2_convert_context *cvcontext);

       void pcre2_convert_context_free(pcre2_convert_context *cvcontext);

       int pcre2_set_glob_escape(pcre2_convert_context *cvcontext,
         uint32_t escape_char);

       int pcre2_set_glob_separator(pcre2_convert_context *cvcontext,
         uint32_t separator_char);

       int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, PCRE2_UCHAR **buffer,
         PCRE2_SIZE *blength, pcre2_convert_context *cvcontext);

       void pcre2_converted_pattern_free(PCRE2_UCHAR *converted_pattern);

       These  functions  provide  a  way of converting non-PCRE2 patterns into
       patterns that can be processed by pcre2_compile(). This facility is ex-
       perimental and may be changed in future releases. At  present,  "globs"
       and  POSIX  basic  and  extended patterns can be converted. Details are
       given in the pcre2convert documentation.


PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

       There are three PCRE2 libraries, supporting 8-bit, 16-bit,  and  32-bit
       code  units,  respectively.  However,  there  is  just one header file,
       pcre2.h.  This contains the function prototypes and  other  definitions
       for all three libraries. One, two, or all three can be installed simul-
       taneously.  On  Unix-like  systems the libraries are called libpcre2-8,
       libpcre2-16, and libpcre2-32, and they can also co-exist with the orig-
       inal PCRE libraries.  Every PCRE2 function  comes  in  three  different
       forms, one for each library, for example:

         pcre2_compile_8()
         pcre2_compile_16()
         pcre2_compile_32()

       There are also three different sets of data types:

         PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
         PCRE2_SPTR8,  PCRE2_SPTR16,  PCRE2_SPTR32

       The  UCHAR  types define unsigned code units of the appropriate widths.
       For example, PCRE2_UCHAR16 is usually defined as `uint16_t'.  The  SPTR
       types are pointers to constants of the equivalent UCHAR types, that is,
       they are pointers to vectors of unsigned code units.

       Character  strings  are  passed  to a PCRE2 library as sequences of un-
       signed integers in code units of the appropriate width. The length of a
       string may be given as a number of code units, or  the  string  may  be
       specified as zero-terminated.

       Many  applications use only one code unit width. For their convenience,
       macros are defined whose names are the generic forms such as pcre2_com-
       pile() and  PCRE2_SPTR.  These  macros  use  the  value  of  the  macro
       PCRE2_CODE_UNIT_WIDTH  to generate the appropriate width-specific func-
       tion and macro names.  PCRE2_CODE_UNIT_WIDTH is not defined by default.
       An application must define it to be  8,  16,  or  32  before  including
       pcre2.h in order to make use of the generic names.

       Applications  that use more than one code unit width can be linked with
       more than one PCRE2 library, but must define  PCRE2_CODE_UNIT_WIDTH  to
       be  0  before  including pcre2.h, and then use the real function names.
       Any code that is to be included in an environment where  the  value  of
       PCRE2_CODE_UNIT_WIDTH  is  unknown  should  also  use the real function
       names. (Unfortunately, it is not possible in C code to save and restore
       the value of a macro.)

       If PCRE2_CODE_UNIT_WIDTH is not defined  before  including  pcre2.h,  a
       compiler error occurs.

       When  using  multiple  libraries  in an application, you must take care
       when processing any particular pattern to use  only  functions  from  a
       single  library.   For example, if you want to run a match using a pat-
       tern that was compiled with pcre2_compile_16(), you  must  do  so  with
       pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().

       In  the  function summaries above, and in the rest of this document and
       other PCRE2 documents, functions and data  types  are  described  using
       their generic names, without the _8, _16, or _32 suffix.


PCRE2 API OVERVIEW

       PCRE2  has  its  own  native  API, which is described in this document.
       There are also some wrapper functions for the 8-bit library that corre-
       spond to the POSIX regular expression API, but they do not give  access
       to  all  the  functionality of PCRE2 and they are not thread-safe. They
       are described in the pcre2posix documentation. Both these APIs define a
       set of C function calls.

       The native API C data types, function prototypes,  option  values,  and
       error codes are defined in the header file pcre2.h, which also contains
       definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release
       numbers  for the library. Applications can use these to include support
       for different releases of PCRE2.

       In a Windows environment, if you want to statically link an application
       program against a non-dll PCRE2 library, you must  define  PCRE2_STATIC
       before including pcre2.h.

       The  functions pcre2_compile() and pcre2_match() are used for compiling
       and matching regular expressions in a Perl-compatible manner. A  sample
       program that demonstrates the simplest way of using them is provided in
       the file called pcre2demo.c in the PCRE2 source distribution. A listing
       of  this  program  is  given  in  the  pcre2demo documentation, and the
       pcre2sample documentation describes how to compile and run it.

       The compiling and matching functions recognize various options that are
       passed as bits in an options argument. There are also some more compli-
       cated parameters such as custom memory  management  functions  and  re-
       source  limits  that  are  passed  in "contexts" (which are just memory
       blocks, described below). Simple applications do not need to  make  use
       of contexts.

       Just-in-time  (JIT)  compiler  support  is an optional feature of PCRE2
       that can be built in  appropriate  hardware  environments.  It  greatly
       speeds  up  the matching performance of many patterns. Programs can re-
       quest that it be used if available by calling pcre2_jit_compile() after
       a pattern has been successfully compiled by pcre2_compile(). This  does
       nothing if JIT support is not available.

       More  complicated  programs  might  need  to make use of the specialist
       functions   pcre2_jit_stack_create(),    pcre2_jit_stack_free(),    and
       pcre2_jit_stack_assign()  in order to control the JIT code's memory us-
       age.

       JIT matching is automatically used by pcre2_match() if it is available,
       unless the PCRE2_NO_JIT option is set. There is also a direct interface
       for JIT matching, which gives improved performance at  the  expense  of
       less  sanity  checking. The JIT-specific functions are discussed in the
       pcre2jit documentation.

       A second matching function, pcre2_dfa_match(), which is  not  Perl-com-
       patible,  is  also  provided.  This  uses a different algorithm for the
       matching. The alternative algorithm finds all possible  matches  (at  a
       given  point  in  the subject), and scans the subject just once (unless
       there are lookaround assertions). However, this algorithm does not  re-
       turn  captured substrings. A description of the two matching algorithms
       and their advantages and disadvantages is given  in  the  pcre2matching
       documentation. There is no JIT support for pcre2_dfa_match().

       In  addition  to  the  main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that has been matched by pcre2_match(). They are:

         pcre2_substring_copy_byname()
         pcre2_substring_copy_bynumber()
         pcre2_substring_get_byname()
         pcre2_substring_get_bynumber()
         pcre2_substring_list_get()
         pcre2_substring_length_byname()
         pcre2_substring_length_bynumber()
         pcre2_substring_nametable_scan()
         pcre2_substring_number_from_name()

       pcre2_substring_free() and pcre2_substring_list_free()  are  also  pro-
       vided,  to  free  memory used for extracted strings. If either of these
       functions is called with a NULL argument, the function returns  immedi-
       ately without doing anything.

       The  function  pcre2_substitute()  can be called to match a pattern and
       return a copy of the subject string with substitutions for  parts  that
       were matched.

       Functions  whose  names begin with pcre2_serialize_ are used for saving
       compiled patterns on disc or elsewhere, and reloading them later.

       Finally, there are functions for finding out information about  a  com-
       piled  pattern  (pcre2_pattern_info()) and about the configuration with
       which PCRE2 was built (pcre2_config()).

       Functions with names ending with _free() are used  for  freeing  memory
       blocks  of  various  sorts.  In all cases, if one of these functions is
       called with a NULL argument, it does nothing.


STRING LENGTHS AND OFFSETS

       The PCRE2 API uses string lengths and  offsets  into  strings  of  code
       units  in  several  places. These values are always of type PCRE2_SIZE,
       which is an unsigned integer type, currently always defined as  size_t.
       The  largest  value  that  can  be  stored  in  such  a  type  (that is
       ~(PCRE2_SIZE)0) is reserved as a special indicator for  zero-terminated
       strings  and  unset offsets.  Therefore, the longest string that can be
       handled is one less than this maximum. Note that string lengths are al-
       ways given in code units. Only in the 8-bit library is  such  a  length
       the same as the number of bytes in the string.


NEWLINES

       PCRE2 supports five different conventions for indicating line breaks in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each of the first three conventions is used by at least  one  operating
       system as its standard newline sequence. When PCRE2 is built, a default
       can be specified.  If it is not, the default is set to LF, which is the
       Unix standard. However, the newline convention can be changed by an ap-
       plication  when calling pcre2_compile(), or it can be specified by spe-
       cial text at the start of the pattern itself; this overrides any  other
       settings.  See the pcre2pattern page for details of the special charac-
       ter sequences.

       In the PCRE2 documentation the word "newline"  is  used  to  mean  "the
       character or pair of characters that indicate a line break". The choice
       of  newline convention affects the handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre2_match() options below.

       The  choice of newline convention does not affect the interpretation of
       the \n or \r escape sequences, nor does it affect what \R matches; this
       has its own separate convention.


MULTITHREADING

       In a multithreaded application it is important to keep  thread-specific
       data  separate  from data that can be shared between threads. The PCRE2
       library code itself is thread-safe: it contains  no  static  or  global
       variables. The API is designed to be fairly simple for non-threaded ap-
       plications  while at the same time ensuring that multithreaded applica-
       tions can use it.

       There are several different blocks of data that are used to pass infor-
       mation between the application and the PCRE2 libraries.

   The compiled pattern

       A pointer to the compiled form of a pattern is  returned  to  the  user
       when pcre2_compile() is successful. The data in the compiled pattern is
       fixed,  and  does not change when the pattern is matched. Therefore, it
       is thread-safe, that is, the same compiled pattern can be used by  more
       than one thread simultaneously. For example, an application can compile
       all its patterns at the start, before forking off multiple threads that
       use  them.  However,  if the just-in-time (JIT) optimization feature is
       being used, it needs separate memory stack areas for each  thread.  See
       the pcre2jit documentation for more details.

       In  a more complicated situation, where patterns are compiled only when
       they are first needed, but are still shared between  threads,  pointers
       to  compiled  patterns  must  be protected from simultaneous writing by
       multiple threads. This is somewhat tricky to do correctly. If you  know
       that  writing  to  a pointer is atomic in your environment, you can use
       logic like this:

         Get a read-only (shared) lock (mutex) for pointer
         if (pointer == NULL)
           {
           Get a write (unique) lock for pointer
           if (pointer == NULL) pointer = pcre2_compile(...
           }
         Release the lock
         Use pointer in pcre2_match()

       Of course, testing for compilation errors should also  be  included  in
       the code.

       The  reason  for checking the pointer a second time is as follows: Sev-
       eral threads may have acquired the shared lock and tested  the  pointer
       for being NULL, but only one of them will be given the write lock, with
       the  rest kept waiting. The winning thread will compile the pattern and
       store the result.  After this thread releases the write  lock,  another
       thread  will  get it, and if it does not retest pointer for being NULL,
       will recompile the pattern and overwrite the pointer, creating a memory
       leak and possibly causing other issues.

       In an environment where writing to a pointer may  not  be  atomic,  the
       above  logic  is not sufficient. The thread that is doing the compiling
       may be descheduled after writing only part of the pointer, which  could
       cause  other  threads  to use an invalid value. Instead of checking the
       pointer itself, a separate "pointer is valid" flag (that can be updated
       atomically) must be used:

         Get a read-only (shared) lock (mutex) for pointer
         if (!pointer_is_valid)
           {
           Get a write (unique) lock for pointer
           if (!pointer_is_valid)
             {
             pointer = pcre2_compile(...
             pointer_is_valid = TRUE
             }
           }
         Release the lock
         Use pointer in pcre2_match()

       If JIT is being used, but the JIT compilation is not being done immedi-
       ately (perhaps waiting to see if the pattern  is  used  often  enough),
       similar  logic  is required. JIT compilation updates a value within the
       compiled code block, so a thread must gain unique write access  to  the
       pointer     before    calling    pcre2_jit_compile().    Alternatively,
       pcre2_code_copy() or pcre2_code_copy_with_tables() can be used  to  ob-
       tain  a  private  copy of the compiled code before calling the JIT com-
       piler.

   Context blocks

       The next main section below introduces the idea of "contexts" in  which
       PCRE2 functions are called. A context is nothing more than a collection
       of parameters that control the way PCRE2 operates. Grouping a number of
       parameters together in a context is a convenient way of passing them to
       a  PCRE2  function without using lots of arguments. The parameters that
       are stored in contexts are in some sense  "advanced  features"  of  the
       API. Many straightforward applications will not need to use contexts.

       In a multithreaded application, if the parameters in a context are val-
       ues  that  are  never  changed, the same context can be used by all the
       threads. However, if any thread needs to change any value in a context,
       it must make its own thread-specific copy.

   Match blocks

       The matching functions need a block of memory for storing  the  results
       of a match. This includes details of what was matched, as well as addi-
       tional  information  such as the name of a (*MARK) setting. Each thread
       must provide its own copy of this memory.


PCRE2 CONTEXTS

       Some PCRE2 functions have a lot of parameters, many of which  are  used
       only  by  specialist  applications,  for example, those that use custom
       memory management or non-standard character tables.  To  keep  function
       argument  lists  at a reasonable size, and at the same time to keep the
       API extensible, "uncommon" parameters are passed to  certain  functions
       in  a  context instead of directly. A context is just a block of memory
       that holds the parameter values.  Applications that do not need to  ad-
       just any of the context parameters can pass NULL when a context pointer
       is required.

       There  are  three different types of context: a general context that is
       relevant for several PCRE2 operations, a compile-time  context,  and  a
       match-time context.

   The general context

       At  present,  this context just contains pointers to (and data for) ex-
       ternal memory management functions that are called from several  places
       in  the  PCRE2  library.  The  context  is  named `general' rather than
       specifically `memory' because in future other fields may be  added.  If
       you  do not want to supply your own custom memory management functions,
       you do not need to bother with a general context. A general context  is
       created by:

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       The  two  function pointers specify custom memory management functions,
       whose prototypes are:

         void *private_malloc(PCRE2_SIZE, void *);
         void  private_free(void *, void *);

       Whenever code in PCRE2 calls these functions, the final argument is the
       value of memory_data. Either of the first two arguments of the creation
       function may be NULL, in which case the system memory management  func-
       tions  malloc()  and free() are used. (This is not currently useful, as
       there are no other fields in a general context,  but  in  future  there
       might  be.)  The private_malloc() function is used (if supplied) to ob-
       tain memory for storing the context, and all three values are saved  as
       part of the context.

       Whenever  PCRE2  creates a data block of any kind, the block contains a
       pointer to the free() function that matches the malloc() function  that
       was  used.  When  the  time  comes  to free the block, this function is
       called.

       A general context can be copied by calling:

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       The memory used for a general context should be freed by calling:

       void pcre2_general_context_free(pcre2_general_context *gcontext);

       If this function is passed a  NULL  argument,  it  returns  immediately
       without doing anything.

   The compile context

       A  compile context is required if you want to provide an external func-
       tion for stack checking during compilation or  to  change  the  default
       values of any of the following compile-time parameters:

         What \R matches (Unicode newlines or CR, LF, CRLF only)
         PCRE2's character tables
         The newline character sequence
         The compile time nested parentheses limit
         The maximum length of the pattern string
         The extra options bits (none set by default)

       A  compile context is also required if you are using custom memory man-
       agement.  If none of these apply, just pass NULL as the  context  argu-
       ment of pcre2_compile().

       A  compile context is created, copied, and freed by the following func-
       tions:

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       A compile context is created with default values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       The  value  must  be PCRE2_BSR_ANYCRLF, to specify that \R matches only
       CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R  matches  any
       Unicode line ending sequence. The value is used by the JIT compiler and
       by   the   two   interpreted   matching  functions,  pcre2_match()  and
       pcre2_dfa_match().

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const uint8_t *tables);

       The value must be the result of a  call  to  pcre2_maketables(),  whose
       only argument is a general context. This function builds a set of char-
       acter tables in the current locale.

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       As  PCRE2  has developed, almost all the 32 option bits that are avail-
       able in the options argument of pcre2_compile() have been used  up.  To
       avoid  running  out, the compile context contains a set of extra option
       bits which are used for some newer, assumed rarer, options. This  func-
       tion  sets  those bits. It always sets all the bits (either on or off).
       It does not modify any existing setting. The available options are  de-
       fined in the section entitled "Extra compile options" below.

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       This  sets a maximum length, in code units, for any pattern string that
       is compiled with this context. If the pattern is longer,  an  error  is
       generated.   This facility is provided so that applications that accept
       patterns from external sources can limit their size. The default is the
       largest number that a PCRE2_SIZE variable can  hold,  which  is  effec-
       tively unlimited.

       int pcre2_set_max_pattern_compiled_length(
         pcre2_compile_context *ccontext, PCRE2_SIZE value);

       This  sets  a maximum size, in bytes, for the memory needed to hold the
       compiled version of a pattern that is compiled with  this  context.  If
       the  pattern needs more memory, an error is generated. This facility is
       provided so  that  applications  that  accept  patterns  from  external
       sources  can  limit  the  amount of memory they use. The default is the
       largest number that a PCRE2_SIZE variable can  hold,  which  is  effec-
       tively unlimited.

       int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext,
         uint32_t value);

       This  sets  a  maximum length for the number of characters matched by a
       variable-length lookbehind assertion. The default is set when PCRE2  is
       built,  with  the ultimate default being 255, the same as Perl. Lookbe-
       hind assertions without a bounding length are not supported.

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       This specifies which characters or character sequences are to be recog-
       nized as newlines. The value must be one of PCRE2_NEWLINE_CR  (carriage
       return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the
       two-character  sequence  CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any
       of the above), PCRE2_NEWLINE_ANY (any  Unicode  newline  sequence),  or
       PCRE2_NEWLINE_NUL (the NUL character, that is a binary zero).

       A pattern can override the value set in the compile context by starting
       with a sequence such as (*CRLF). See the pcre2pattern page for details.

       When  a  pattern  is  compiled  with  the  PCRE2_EXTENDED  or PCRE2_EX-
       TENDED_MORE option, the newline convention affects the  recognition  of
       the  end  of internal comments starting with #. The value is saved with
       the compiled pattern for subsequent use by the JIT compiler and by  the
       two     interpreted     matching     functions,    pcre2_match()    and
       pcre2_dfa_match().

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       This parameter adjusts the limit, set  when  PCRE2  is  built  (default
       250),  on  the  depth  of  parenthesis nesting in a pattern. This limit
       stops rogue patterns using up too much system  stack  when  being  com-
       piled.  The limit applies to parentheses of all kinds, not just captur-
       ing parentheses.

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);

       There is at least one application that runs PCRE2 in threads with  very
       limited  system  stack,  where running out of stack is to be avoided at
       all costs. The parenthesis limit above cannot take account of how  much
       stack  is  actually  available during compilation. For a finer control,
       you can supply a  function  that  is  called  whenever  pcre2_compile()
       starts  to compile a parenthesized part of a pattern. This function can
       check the actual stack size (or anything else  that  it  wants  to,  of
       course).

       The  first  argument to the callout function gives the current depth of
       nesting, and the second is user data that is set up by the  last  argu-
       ment   of  pcre2_set_compile_recursion_guard().  The  callout  function
       should return zero if all is well, or non-zero to force an error.

   The match context

       A match context is required if you want to:

         Set up a callout function
         Set an offset limit for matching an unanchored pattern
         Change the limit on the amount of heap used when matching
         Change the backtracking match limit
         Change the backtracking depth limit
         Set custom memory management specifically for the match

       If none of these apply, just pass  NULL  as  the  context  argument  of
       pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

       A  match  context  is created, copied, and freed by the following func-
       tions:

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       A match context is created with  default  values  for  its  parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       This  sets  up a callout function for PCRE2 to call at specified points
       during a matching operation. Details are given in the pcre2callout doc-
       umentation.

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       This sets up a callout function for PCRE2 to call after each  substitu-
       tion made by pcre2_substitute(). Details are given in the section enti-
       tled "Creating a new string with substitutions" below.

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       The  offset_limit parameter limits how far an unanchored search can ad-
       vance in the subject string. The  default  value  is  PCRE2_UNSET.  The
       pcre2_match()  and  pcre2_dfa_match()  functions return PCRE2_ERROR_NO-
       MATCH if a match with a starting point before or at the given offset is
       not found. The pcre2_substitute() function makes no more substitutions.

       For example, if the pattern /abc/ is matched against "123abc"  with  an
       offset  limit  less  than 3, the result is PCRE2_ERROR_NOMATCH. A match
       can never be  found  if  the  startoffset  argument  of  pcre2_match(),
       pcre2_dfa_match(),  or  pcre2_substitute()  is  greater than the offset
       limit set in the match context.

       When using this facility, you must set the  PCRE2_USE_OFFSET_LIMIT  op-
       tion when calling pcre2_compile() so that when JIT is in use, different
       code  can  be  compiled. If a match is started with a non-default match
       limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is generated.

       The offset limit facility can be used to track progress when  searching
       large  subject  strings or to limit the extent of global substitutions.
       See also the PCRE2_FIRSTLINE option, which requires a  match  to  start
       before  or  at  the first newline that follows the start of matching in
       the subject. If this is set with an offset limit, a match must occur in
       the first line and also  within  the  offset  limit.  In  other  words,
       whichever limit comes first is used.

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The heap_limit parameter specifies, in units of kibibytes (1024 bytes),
       the  maximum  amount  of heap memory that pcre2_match() may use to hold
       backtracking information when running an interpretive match. This limit
       also applies to pcre2_dfa_match(), which may use the heap when process-
       ing patterns with a lot of nested pattern recursion or  lookarounds  or
       atomic groups. This limit does not apply to matching with the JIT opti-
       mization,  which  has  its  own  memory  control  arrangements (see the
       pcre2jit documentation for more details). If the limit is reached,  the
       negative  error  code  PCRE2_ERROR_HEAPLIMIT  is  returned. The default
       limit can be set when PCRE2 is built; if it is not, the default is  set
       very large and is essentially unlimited.

       A value for the heap limit may also be supplied by an item at the start
       of a pattern of the form

         (*LIMIT_HEAP=ddd)

       where  ddd  is a decimal number. However, such a setting is ignored un-
       less ddd is less than the limit set by the caller of pcre2_match()  or,
       if no such limit is set, less than the default.

       The  pcre2_match() function always needs some heap memory, so setting a
       value of zero guarantees a "heap limit exceeded" error. Details of  how
       pcre2_match()  uses  the  heap are given in the pcre2perform documenta-
       tion.

       For pcre2_dfa_match(), a vector on the system stack is used  when  pro-
       cessing  pattern recursions, lookarounds, or atomic groups, and only if
       this is not big enough is heap memory used. In  this  case,  setting  a
       value of zero disables the use of the heap.

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The match_limit parameter provides a means of preventing PCRE2 from us-
       ing  up  too many computing resources when processing patterns that are
       not going to match, but which have a very large number of possibilities
       in their search trees. The classic  example  is  a  pattern  that  uses
       nested unlimited repeats.

       There  is an internal counter in pcre2_match() that is incremented each
       time round its main matching loop. If  this  value  reaches  the  match
       limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT.
       This  has  the  effect  of limiting the amount of backtracking that can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string. This limit  also  applies
       to pcre2_dfa_match(), though the counting is done in a different way.

       When  pcre2_match()  is  called  with  a  pattern that was successfully
       processed by pcre2_jit_compile(), the way in which matching is executed
       is entirely different. However, there is still the possibility of  run-
       away matching that goes on for a very long time, and so the match_limit
       value  is  also used in this case (but in a different way) to limit how
       long the matching can continue.

       The default value for the limit can be set when PCRE2 is built; the de-
       fault is 10 million, which handles all but the most  extreme  cases.  A
       value  for the match limit may also be supplied by an item at the start
       of a pattern of the form

         (*LIMIT_MATCH=ddd)

       where ddd is a decimal number. However, such a setting is  ignored  un-
       less  ddd  is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);

       This  parameter  limits   the   depth   of   nested   backtracking   in
       pcre2_match().   Each time a nested backtracking point is passed, a new
       memory frame is used to remember the state of matching at  that  point.
       Thus,  this  parameter  indirectly  limits the amount of memory that is
       used in a match. However, because the size of each memory frame depends
       on the number of capturing parentheses, the actual memory limit  varies
       from  pattern to pattern. This limit was more useful in versions before
       10.30, where function recursion was used for backtracking.

       The depth limit is not relevant, and is ignored, when matching is  done
       using JIT compiled code. However, it is supported by pcre2_dfa_match(),
       which  uses it to limit the depth of nested internal recursive function
       calls that implement atomic groups, lookaround assertions, and  pattern
       recursions. This limits, indirectly, the amount of system stack that is
       used.  It  was  more useful in versions before 10.32, when stack memory
       was used for local workspace vectors for recursive function calls. From
       version 10.32, only local variables are allocated on the stack  and  as
       each call uses only a few hundred bytes, even a small stack can support
       quite a lot of recursion.

       If  the depth of internal recursive function calls is great enough, lo-
       cal workspace vectors are allocated on the heap from version 10.32  on-
       wards,  so  the  depth  limit also indirectly limits the amount of heap
       memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when
       matched to a very long string using pcre2_dfa_match(), can use a  great
       deal  of memory. However, it is probably better to limit heap usage di-
       rectly by calling pcre2_set_heap_limit().

       The default value for the depth limit can be set when PCRE2  is  built;
       if  it  is not, the default is set to the same value as the default for
       the  match  limit.   If  the  limit  is  exceeded,   pcre2_match()   or
       pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth
       limit  may also be supplied by an item at the start of a pattern of the
       form

         (*LIMIT_DEPTH=ddd)

       where ddd is a decimal number. However, such a setting is  ignored  un-
       less  ddd  is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.


CHECKING BUILD-TIME OPTIONS

       int pcre2_config(uint32_t what, void *where);

       The function pcre2_config() makes it possible for  a  PCRE2  client  to
       find  the  value  of  certain  configuration parameters and to discover
       which optional features have been compiled into the PCRE2 library.  The
       pcre2build documentation has more details about these features.

       The  first  argument  for pcre2_config() specifies which information is
       required. The second argument is a pointer to memory into which the in-
       formation is placed. If NULL is passed, the function returns the amount
       of memory that is needed for the requested information. For calls  that
       return  numerical  values, the value is in bytes; when requesting these
       values, where should point to appropriately aligned memory.  For  calls
       that  return  strings,  the required length is given in code units, not
       counting the terminating zero.

       When requesting information, the returned value from pcre2_config()  is
       non-negative  on success, or the negative error code PCRE2_ERROR_BADOP-
       TION if the value in the first argument is not recognized. The  follow-
       ing information is available:

         PCRE2_CONFIG_BSR

       The  output  is a uint32_t integer whose value indicates what character
       sequences the \R  escape  sequence  matches  by  default.  A  value  of
       PCRE2_BSR_UNICODE  means  that  \R  matches any Unicode line ending se-
       quence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF,
       or CRLF. The default can be overridden when a pattern is compiled.

         PCRE2_CONFIG_COMPILED_WIDTHS

       The output is a uint32_t integer whose lower bits indicate  which  code
       unit  widths  were  selected  when PCRE2 was built. The 1-bit indicates
       8-bit support, and the 2-bit and 4-bit indicate 16-bit and 32-bit  sup-
       port, respectively.

         PCRE2_CONFIG_DEPTHLIMIT

       The  output  is a uint32_t integer that gives the default limit for the
       depth of nested backtracking in pcre2_match() or the  depth  of  nested
       recursions,  lookarounds,  and atomic groups in pcre2_dfa_match(). Fur-
       ther details are given with pcre2_set_depth_limit() above.

         PCRE2_CONFIG_HEAPLIMIT

       The output is a uint32_t integer that gives, in kibibytes, the  default
       limit   for  the  amount  of  heap  memory  used  by  pcre2_match()  or
       pcre2_dfa_match().     Further     details     are      given      with
       pcre2_set_heap_limit() above.

         PCRE2_CONFIG_JIT

       The  output  is  a  uint32_t  integer that is set to one if support for
       just-in-time compiling is included in the library; otherwise it is  set
       to zero. Note that having the support in the library does not guarantee
       that  JIT will be used for any given match. See the pcre2jit documenta-
       tion for more details.

         PCRE2_CONFIG_JITTARGET

       The where argument should point to a buffer that is at  least  48  code
       units  long.  (The  exact  length  required  can  be  found  by calling
       pcre2_config() with where set to NULL.) The buffer  is  filled  with  a
       string  that  contains  the  name of the architecture for which the JIT
       compiler is configured, for example "x86 32bit  (little  endian  +  un-
       aligned)".  If  JIT  support is not available, PCRE2_ERROR_BADOPTION is
       returned, otherwise the number of code units used is returned. This  is
       the length of the string, plus one unit for the terminating zero.

         PCRE2_CONFIG_LINKSIZE

       The output is a uint32_t integer that contains the number of bytes used
       for  internal  linkage  in  compiled regular expressions. When PCRE2 is
       configured, the value can be set to 2, 3, or 4, with the default  being
       2.  This is the value that is returned by pcre2_config(). However, when
       the 16-bit library is compiled, a value of 3 is rounded up  to  4,  and
       when  the  32-bit  library  is compiled, internal linkages always use 4
       bytes, so the configured value is not relevant.

       The default value of 2 for the 8-bit and 16-bit libraries is sufficient
       for all but the most massive patterns, since it allows the size of  the
       compiled  pattern  to  be  up  to 65535 code units. Larger values allow
       larger regular expressions to be compiled by those two  libraries,  but
       at the expense of slower matching.

         PCRE2_CONFIG_MATCHLIMIT

       The output is a uint32_t integer that gives the default match limit for
       pcre2_match().  Further  details are given with pcre2_set_match_limit()
       above.

         PCRE2_CONFIG_NEWLINE

       The output is a uint32_t integer  whose  value  specifies  the  default
       character  sequence that is recognized as meaning "newline". The values
       are:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       The default should normally correspond to  the  standard  sequence  for
       your operating system.

         PCRE2_CONFIG_NEVER_BACKSLASH_C

       The  output  is  a uint32_t integer that is set to one if the use of \C
       was permanently disabled when PCRE2 was built; otherwise it is  set  to
       zero.

         PCRE2_CONFIG_PARENSLIMIT

       The  output is a uint32_t integer that gives the maximum depth of nest-
       ing of parentheses (of any kind) in a pattern. This limit is imposed to
       cap the amount of system stack used when a pattern is compiled.  It  is
       specified  when PCRE2 is built; the default is 250. This limit does not
       take into account the stack that may already be used by the calling ap-
       plication.  For  finer  control  over  compilation  stack  usage,   see
       pcre2_set_compile_recursion_guard().

         PCRE2_CONFIG_STACKRECURSE

       This parameter is obsolete and should not be used in new code. The out-
       put is a uint32_t integer that is always set to zero.

         PCRE2_CONFIG_TABLES_LENGTH

       The output is a uint32_t integer that gives the length of PCRE2's char-
       acter  processing  tables in bytes. For details of these tables see the
       section on locale support below.

         PCRE2_CONFIG_UNICODE_VERSION

       The where argument should point to a buffer that is at  least  24  code
       units  long.  (The  exact  length  required  can  be  found  by calling
       pcre2_config() with where set to NULL.)  If  PCRE2  has  been  compiled
       without  Unicode  support,  the buffer is filled with the text "Unicode
       not supported". Otherwise, the Unicode  version  string  (for  example,
       "8.0.0")  is  inserted. The number of code units used is returned. This
       is the length of the string plus one unit for the terminating zero.

         PCRE2_CONFIG_UNICODE

       The output is a uint32_t integer that is set to one if Unicode  support
       is  available; otherwise it is set to zero. Unicode support implies UTF
       support.

         PCRE2_CONFIG_VERSION

       The where argument should point to a buffer that is at  least  24  code
       units  long.  (The  exact  length  required  can  be  found  by calling
       pcre2_config() with where set to NULL.) The buffer is filled  with  the
       PCRE2 version string, zero-terminated. The number of code units used is
       returned. This is the length of the string plus one unit for the termi-
       nating zero.


COMPILING A PATTERN

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       The  pcre2_compile() function compiles a pattern into an internal form.
       The pattern is defined by a pointer to a string of  code  units  and  a
       length in code units. If the pattern is zero-terminated, the length can
       be  specified  as  PCRE2_ZERO_TERMINATED. A NULL pattern pointer with a
       length of zero is treated as an empty  string  (NULL  with  a  non-zero
       length  causes  an  error  return). The function returns a pointer to a
       block of memory that contains the compiled pattern and related data, or
       NULL if an error occurred.

       If the compile context argument ccontext is NULL, memory for  the  com-
       piled  pattern  is  obtained  by calling malloc(). Otherwise, it is ob-
       tained from the same memory function that was used for the compile con-
       text. The caller must free the memory by calling pcre2_code_free() when
       it is no longer needed.  If pcre2_code_free() is called with a NULL ar-
       gument, it returns immediately, without doing anything.

       The function pcre2_code_copy() makes a copy of the compiled code in new
       memory, using the same memory allocator as was used for  the  original.
       However,  if  the  code has been processed by the JIT compiler (see be-
       low), the JIT information cannot be copied (because it is  position-de-
       pendent).   The  new copy can initially be used only for non-JIT match-
       ing, though it can be passed to  pcre2_jit_compile()  if  required.  If
       pcre2_code_copy() is called with a NULL argument, it returns NULL.

       The pcre2_code_copy() function provides a way for individual threads in
       a  multithreaded  application  to acquire a private copy of shared com-
       piled code.  However, it does not make a copy of the  character  tables
       used  by  the compiled pattern; the new pattern code points to the same
       tables as the original code.  (See "Locale Support" below  for  details
       of  these  character  tables.) In many applications the same tables are
       used throughout, so this behaviour is appropriate. Nevertheless,  there
       are occasions when a copy of a compiled pattern and the relevant tables
       are  needed.  The pcre2_code_copy_with_tables() provides this facility.
       Copies of both the code and the tables are  made,  with  the  new  code
       pointing  to the new tables. The memory for the new tables is automati-
       cally freed when pcre2_code_free() is called for the new  copy  of  the
       compiled  code.  If pcre2_code_copy_with_tables() is called with a NULL
       argument, it returns NULL.

       NOTE: When one of the matching functions is  called,  pointers  to  the
       compiled pattern and the subject string are set in the match data block
       so  that  they  can be referenced by the substring extraction functions
       after a successful match.  After running a match, you must not  free  a
       compiled  pattern or a subject string until after all operations on the
       match data block have taken place, unless, in the case of  the  subject
       string,  you  have used the PCRE2_COPY_MATCHED_SUBJECT option, which is
       described in the section entitled "Option bits for  pcre2_match()"  be-
       low.

       The  options argument for pcre2_compile() contains various bit settings
       that affect the compilation. It should be zero if none of them are  re-
       quired.  The  available  options  are described below. Some of them (in
       particular, those that are compatible with Perl,  but  some  others  as
       well)  can  also  be set and unset from within the pattern (see the de-
       tailed description in the pcre2pattern documentation).

       For those options that can be different in different parts of the  pat-
       tern,  the contents of the options argument specifies their settings at
       the start of compilation. The  PCRE2_ANCHORED,  PCRE2_ENDANCHORED,  and
       PCRE2_NO_UTF_CHECK  options  can be set at the time of matching as well
       as at compile time.

       Some additional options and less frequently required compile-time para-
       meters (for example, the newline setting) can be provided in a  compile
       context (as described above).

       If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme-
       diately.  Otherwise,  the  variables to which these point are set to an
       error code and an offset (number of code units) within the pattern, re-
       spectively, when pcre2_compile() returns NULL because a compilation er-
       ror has occurred.

       There are nearly 100 positive error codes that pcre2_compile() may  re-
       turn  if it finds an error in the pattern. There are also some negative
       error codes that are used for invalid UTF strings when validity  check-
       ing  is  in  force.  These  are  the same as given by pcre2_match() and
       pcre2_dfa_match(), and are described in the pcre2unicode documentation.
       There is no separate documentation for the positive  error  codes,  be-
       cause  the  textual  error  messages  that  are obtained by calling the
       pcre2_get_error_message() function (see "Obtaining a textual error mes-
       sage" below) should be  self-explanatory.  Macro  names  starting  with
       PCRE2_ERROR_  are defined for both positive and negative error codes in
       pcre2.h. When compilation is successful errorcode is  set  to  a  value
       that  returns  the message "no error" if passed to pcre2_get_error_mes-
       sage().

       The value returned in erroroffset is an indication of where in the pat-
       tern an error occurred. When there is no error,  zero  is  returned.  A
       non-zero  value  is  not  necessarily the furthest point in the pattern
       that was read. For example, after the error  "lookbehind  assertion  is
       not  fixed length", the error offset points to the start of the failing
       assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of
       the first code unit of the failing character.

       Some errors are not detected until the whole pattern has been  scanned;
       in  these  cases,  the offset passed back is the length of the pattern.
       Note that the offset is in code units, not characters, even  in  a  UTF
       mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char-
       acter.

       This  code  fragment shows a typical straightforward call to pcre2_com-
       pile():

         pcre2_code *re;
         PCRE2_SIZE erroffset;
         int errorcode;
         re = pcre2_compile(
           "^A.*Z",                /* the pattern */
           PCRE2_ZERO_TERMINATED,  /* the pattern is zero-terminated */
           0,                      /* default options */
           &errorcode,             /* for error code */
           &erroffset,             /* for error offset */
           NULL);                  /* no compile context */


   Main compile options

       The following names for option bits are defined in the  pcre2.h  header
       file:

         PCRE2_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE2_ALLOW_EMPTY_CLASS

       By default, for compatibility with Perl, a closing square bracket  that
       immediately  follows  an opening one is treated as a data character for
       the class. When  PCRE2_ALLOW_EMPTY_CLASS  is  set,  it  terminates  the
       class, which therefore contains no characters and so can never match.

         PCRE2_ALT_BSUX

       This  option  request  alternative  handling of three escape sequences,
       which makes PCRE2's behaviour more like  ECMAscript  (aka  JavaScript).
       When it is set:

       (1) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (2) \u matches a lower case "u" character unless it is followed by four
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, \u causes a compile time  error  (Perl
       uses it to upper case the following character).

       (3)  \x matches a lower case "x" character unless it is followed by two
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to  match. By default, as in Perl, a hexadecimal number is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

       ECMAscript 6 added additional functionality to \u. This can be accessed
       using the PCRE2_EXTRA_ALT_BSUX extra option  (see  "Extra  compile  op-
       tions" below).  Note that this alternative escape handling applies only
       to  patterns.  Neither  of  these options affects the processing of re-
       placement strings passed to pcre2_substitute().

         PCRE2_ALT_CIRCUMFLEX

       In  multiline  mode  (when  PCRE2_MULTILINE  is  set),  the  circumflex
       metacharacter  matches at the start of the subject (unless PCRE2_NOTBOL
       is set), and also after any internal  newline.  However,  it  does  not
       match after a newline at the end of the subject, for compatibility with
       Perl.  If  you want a multiline circumflex also to match after a termi-
       nating newline, you must set PCRE2_ALT_CIRCUMFLEX.

         PCRE2_ALT_VERBNAMES

       By default, for compatibility with Perl, the name in any verb  sequence
       such  as  (*MARK:NAME)  is any sequence of characters that does not in-
       clude a closing parenthesis. The name is not processed in any way,  and
       it  is  not possible to include a closing parenthesis in the name. How-
       ever, if the PCRE2_ALT_VERBNAMES option is set, normal  backslash  pro-
       cessing  is  applied to verb names and only an unescaped closing paren-
       thesis terminates the name. A closing parenthesis can be included in  a
       name  either  as  \)  or  between  \Q  and \E. If the PCRE2_EXTENDED or
       PCRE2_EXTENDED_MORE option is set with  PCRE2_ALT_VERBNAMES,  unescaped
       whitespace  in verb names is skipped and #-comments are recognized, ex-
       actly as in the rest of the pattern.

         PCRE2_AUTO_CALLOUT

       If this bit  is  set,  pcre2_compile()  automatically  inserts  callout
       items,  all  with  number 255, before each pattern item, except immedi-
       ately before or after an explicit callout in the pattern.  For  discus-
       sion of the callout facility, see the pcre2callout documentation.

         PCRE2_CASELESS

       If  this  bit is set, letters in the pattern match both upper and lower
       case letters in the subject. It is equivalent to Perl's /i option,  and
       it  can be changed within a pattern by a (?i) option setting. If either
       PCRE2_UTF or PCRE2_UCP is set, Unicode  properties  are  used  for  all
       characters  with more than one other case, and for all characters whose
       code points are greater than U+007F. Note  that  there  are  two  ASCII
       characters, K and S, that, in addition to their lower case ASCII equiv-
       alents,  are case-equivalent with U+212A (Kelvin sign) and U+017F (long
       S) respectively. If you do not want this case equivalence, you can sup-
       press it by setting PCRE2_EXTRA_CASELESS_RESTRICT.

       For lower valued characters with only one other case, a lookup table is
       used for speed. When neither PCRE2_UTF nor PCRE2_UCP is set,  a  lookup
       table is used for all code points less than 256, and higher code points
       (available only in 16-bit or 32-bit mode) are treated as not having an-
       other case.

         PCRE2_DOLLAR_ENDONLY

       If  this bit is set, a dollar metacharacter in the pattern matches only
       at the end of the subject string. Without this option,  a  dollar  also
       matches  immediately before a newline at the end of the string (but not
       before any other newlines). The PCRE2_DOLLAR_ENDONLY option is  ignored
       if  PCRE2_MULTILINE  is  set.  There is no equivalent to this option in
       Perl, and no way to set it within a pattern.

         PCRE2_DOTALL

       If this bit is set, a dot metacharacter  in  the  pattern  matches  any
       character,  including  one  that  indicates a newline. However, it only
       ever matches one character, even if newlines are coded as CRLF. Without
       this option, a dot does not match when the current position in the sub-
       ject is at a newline. This option is equivalent to  Perl's  /s  option,
       and it can be changed within a pattern by a (?s) option setting. A neg-
       ative  class such as [^a] always matches newline characters, and the \N
       escape sequence always matches a non-newline character, independent  of
       the setting of PCRE2_DOTALL.

         PCRE2_DUPNAMES

       If  this  bit is set, names used to identify capture groups need not be
       unique.  This can be helpful for certain types of pattern  when  it  is
       known  that  only  one instance of the named group can ever be matched.
       There are more details of named capture  groups  below;  see  also  the
       pcre2pattern documentation.

         PCRE2_ENDANCHORED

       If  this  bit is set, the end of any pattern match must be right at the
       end of the string being searched (the "subject string"). If the pattern
       match succeeds by reaching (*ACCEPT), but does not reach the end of the
       subject, the match fails at the current starting point. For  unanchored
       patterns,  a  new  match is then tried at the next starting point. How-
       ever, if the match succeeds by reaching the end of the pattern, but not
       the end of the subject, backtracking occurs and  an  alternative  match
       may be found. Consider these two patterns:

         .(*ACCEPT)|..
         .|..

       If  matched against "abc" with PCRE2_ENDANCHORED set, the first matches
       "c" whereas the second matches "bc". The  effect  of  PCRE2_ENDANCHORED
       can  also  be achieved by appropriate constructs in the pattern itself,
       which is the only way to do it in Perl.

       For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only
       to the first (that is, the  longest)  matched  string.  Other  parallel
       matches,  which are necessarily substrings of the first one, must obvi-
       ously end before the end of the subject.

         PCRE2_EXTENDED

       If this bit is set, most white space characters in the pattern are  to-
       tally  ignored except when escaped, inside a character class, or inside
       a \Q...\E sequence. However, white space  is  not  allowed  within  se-
       quences  such  as  (?> that introduce various parenthesized groups, nor
       within numerical quantifiers such as {1,3}. Ignorable  white  space  is
       permitted  between  an  item  and  a following quantifier and between a
       quantifier and a following + that indicates  possessiveness.  PCRE2_EX-
       TENDED  is equivalent to Perl's /x option, and it can be changed within
       a pattern by a (?x) option setting.

       When PCRE2 is compiled without Unicode support,  PCRE2_EXTENDED  recog-
       nizes  as  white space only those characters with code points less than
       256 that are flagged as white space in its low-character table. The ta-
       ble is normally created by pcre2_maketables(), which uses the isspace()
       function to identify space characters. In most ASCII environments,  the
       relevant  characters  are  those  with code points 0x0009 (tab), 0x000A
       (linefeed), 0x000B (vertical tab), 0x000C (formfeed), 0x000D  (carriage
       return), and 0x0020 (space).

       When PCRE2 is compiled with Unicode support, in addition to these char-
       acters,  five  more Unicode "Pattern White Space" characters are recog-
       nized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E (left-to-
       right mark), U+200F (right-to-left mark), U+2028 (line separator),  and
       U+2029  (paragraph  separator).  This  set of characters is the same as
       recognized by Perl's /x option. Note that the horizontal  and  vertical
       space  characters that are matched by the \h and \v escapes in patterns
       are a much bigger set.

       As well as ignoring most white space, PCRE2_EXTENDED also causes  char-
       acters  between  an  unescaped # outside a character class and the next
       newline, inclusive, to be ignored, which makes it possible  to  include
       comments inside complicated patterns. Note that the end of this type of
       comment  is a literal newline sequence in the pattern; escape sequences
       that happen to represent a newline do not count.

       Which characters are interpreted as newlines can be specified by a set-
       ting in the compile context that is passed to pcre2_compile() or  by  a
       special  sequence at the start of the pattern, as described in the sec-
       tion entitled "Newline conventions" in the pcre2pattern  documentation.
       A default is defined when PCRE2 is built.

         PCRE2_EXTENDED_MORE

       This  option  has  the  effect of PCRE2_EXTENDED, but, in addition, un-
       escaped space and horizontal tab characters are ignored inside a  char-
       acter  class. Note: only these two characters are ignored, not the full
       set of pattern white space characters that are ignored outside a  char-
       acter  class.  PCRE2_EXTENDED_MORE  is equivalent to Perl's /xx option,
       and it can be changed within a pattern by a (?xx) option setting.

         PCRE2_FIRSTLINE

       If this option is set, the start of an unanchored pattern match must be
       before or at the first newline in  the  subject  string  following  the
       start  of  matching, though the matched text may continue over the new-
       line. If startoffset is non-zero, the limiting newline is not necessar-
       ily the first newline in the  subject.  For  example,  if  the  subject
       string is "abc\nxyz" (where \n represents a single-character newline) a
       pattern  match for "yz" succeeds with PCRE2_FIRSTLINE if startoffset is
       greater than 3. See also PCRE2_USE_OFFSET_LIMIT, which provides a  more
       general  limiting  facility.  If  PCRE2_FIRSTLINE is set with an offset
       limit, a match must occur in the first line and also within the  offset
       limit. In other words, whichever limit comes first is used. This option
       has no effect for anchored patterns.

         PCRE2_LITERAL

       If this option is set, all meta-characters in the pattern are disabled,
       and  it is treated as a literal string. Matching literal strings with a
       regular expression engine is not the most efficient way of doing it. If
       you are doing a lot of literal matching and  are  worried  about  effi-
       ciency, you should consider using other approaches. The only other main
       options  that  are  allowed  with  PCRE2_LITERAL  are:  PCRE2_ANCHORED,
       PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT, PCRE2_CASELESS, PCRE2_FIRSTLINE,
       PCRE2_MATCH_INVALID_UTF,  PCRE2_NO_START_OPTIMIZE,  PCRE2_NO_UTF_CHECK,
       PCRE2_UTF,  and  PCRE2_USE_OFFSET_LIMIT.  The  extra  options PCRE2_EX-
       TRA_MATCH_LINE and PCRE2_EXTRA_MATCH_WORD are also supported. Any other
       options cause an error.

         PCRE2_MATCH_INVALID_UTF

       This option forces PCRE2_UTF (see below) and also enables  support  for
       matching  by  pcre2_match() in subject strings that contain invalid UTF
       sequences.  Note, however, that the 16-bit and 32-bit  PCRE2  libraries
       process  strings as sequences of uint16_t or uint32_t code points. They
       cannot find valid UTF sequences within an arbitrary string of bytes un-
       less such sequences are suitably aligned. This  facility  is  not  sup-
       ported  for  DFA matching. For details, see the pcre2unicode documenta-
       tion.

         PCRE2_MATCH_UNSET_BACKREF

       If this option is set,  a  backreference  to  an  unset  capture  group
       matches  an  empty  string (by default this causes the current matching
       alternative to fail).  A pattern such as (\1)(a) succeeds when this op-
       tion is set (assuming it can find an "a" in the  subject),  whereas  it
       fails  by  default,  for  Perl compatibility. Setting this option makes
       PCRE2 behave more like ECMAscript (aka JavaScript).

         PCRE2_MULTILINE

       By default, for the purposes of matching "start of line"  and  "end  of
       line",  PCRE2  treats the subject string as consisting of a single line
       of characters, even if it actually contains  newlines.  The  "start  of
       line"  metacharacter  (^)  matches only at the start of the string, and
       the "end of line" metacharacter ($) matches only  at  the  end  of  the
       string,  or  before a terminating newline (except when PCRE2_DOLLAR_EN-
       DONLY is set). Note, however, that unless PCRE2_DOTALL is set, the "any
       character" metacharacter (.) does not match at a newline.  This  behav-
       iour (for ^, $, and dot) is the same as Perl.

       When  PCRE2_MULTILINE  it is set, the "start of line" and "end of line"
       constructs match immediately following or immediately  before  internal
       newlines  in  the  subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and  it  can  be
       changed within a pattern by a (?m) option setting. Note that the "start
       of line" metacharacter does not match after a newline at the end of the
       subject,  for compatibility with Perl.  However, you can change this by
       setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in  a
       subject  string,  or  no  occurrences  of  ^ or $ in a pattern, setting
       PCRE2_MULTILINE has no effect.

         PCRE2_NEVER_BACKSLASH_C

       This option locks out the use of \C in the pattern that is  being  com-
       piled.   This  escape  can  cause  unpredictable  behaviour in UTF-8 or
       UTF-16 modes, because it may leave the current matching  point  in  the
       middle of a multi-code-unit character. This option may be useful in ap-
       plications that process patterns from external sources. Note that there
       is also a build-time option that permanently locks out the use of \C.

         PCRE2_NEVER_UCP

       This  option  locks  out the use of Unicode properties for handling \B,
       \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as
       described for the PCRE2_UCP option below. In  particular,  it  prevents
       the  creator of the pattern from enabling this facility by starting the
       pattern with (*UCP). This option may be  useful  in  applications  that
       process patterns from external sources. The option combination PCRE_UCP
       and PCRE_NEVER_UCP causes an error.

         PCRE2_NEVER_UTF

       This  option  locks out interpretation of the pattern as UTF-8, UTF-16,
       or UTF-32, depending on which library is in use. In particular, it pre-
       vents the creator of the pattern from switching to  UTF  interpretation
       by  starting  the pattern with (*UTF). This option may be useful in ap-
       plications that process patterns from external sources. The combination
       of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.

         PCRE2_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses in the pattern. Any opening parenthesis that is not followed  by
       ?  behaves as if it were followed by ?: but named parentheses can still
       be used for capturing (and they acquire numbers in the usual way). This
       is the same as Perl's /n option.  Note that, when this option  is  set,
       references  to  capture  groups (backreferences or recursion/subroutine
       calls) may only refer to named groups, though the reference can  be  by
       name or by number.

         PCRE2_NO_AUTO_POSSESS

       If this option is set, it disables "auto-possessification", which is an
       optimization  that,  for example, turns a+b into a++b in order to avoid
       backtracks into a+ that can never be successful. However,  if  callouts
       are  in  use,  auto-possessification means that some callouts are never
       taken. You can set this option if you want the matching functions to do
       a full unoptimized search and run all the callouts, but  it  is  mainly
       provided for testing purposes.

         PCRE2_NO_DOTSTAR_ANCHOR

       If this option is set, it disables an optimization that is applied when
       .*  is  the  first significant item in a top-level branch of a pattern,
       and all the other branches also start with .* or with \A or  \G  or  ^.
       The  optimization  is  automatically disabled for .* if it is inside an
       atomic group or a capture group that is the subject of a backreference,
       or if the pattern contains (*PRUNE) or (*SKIP). When  the  optimization
       is   not   disabled,  such  a  pattern  is  automatically  anchored  if
       PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set
       for any ^ items. Otherwise, the fact that any match must  start  either
       at  the start of the subject or following a newline is remembered. Like
       other optimizations, this can cause callouts to be skipped.

         PCRE2_NO_START_OPTIMIZE

       This is an option whose main effect is at matching time.  It  does  not
       change what pcre2_compile() generates, but it does affect the output of
       the JIT compiler.

       There  are  a  number of optimizations that may occur at the start of a
       match, in order to speed up the process. For example, if  it  is  known
       that  an  unanchored  match must start with a specific code unit value,
       the matching code searches the subject for that value, and fails  imme-
       diately  if it cannot find it, without actually running the main match-
       ing function. This means that a special item such as (*COMMIT)  at  the
       start  of  a  pattern is not considered until after a suitable starting
       point for the match has been found.  Also,  when  callouts  or  (*MARK)
       items  are  in use, these "start-up" optimizations can cause them to be
       skipped if the pattern is never actually used. The  start-up  optimiza-
       tions  are  in effect a pre-scan of the subject that takes place before
       the pattern is run.

       The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly causing performance to suffer,  but  ensuring  that  in  cases
       where  the  result is "no match", the callouts do occur, and that items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position in the subject string.

       Setting PCRE2_NO_START_OPTIMIZE may change the outcome  of  a  matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When  this  is compiled, PCRE2 records the fact that a match must start
       with the character "A". Suppose the subject  string  is  "DEFABC".  The
       start-up  optimization  scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the  pat-
       tern  must  match the current starting position, which in this case, it
       does. However, if the same match is  run  with  PCRE2_NO_START_OPTIMIZE
       set,  the  initial  scan  along the subject string does not happen. The
       first match attempt is run starting  from  "D"  and  when  this  fails,
       (*COMMIT)  prevents any further matches being tried, so the overall re-
       sult is "no match".

       As another start-up optimization makes use of a minimum  length  for  a
       matching subject, which is recorded when possible. Consider the pattern

         (*MARK:1)B(*MARK:2)(X|Y)

       The  minimum  length  for  a match is two characters. If the subject is
       "XXBB", the "starting character" optimization skips "XX", then tries to
       match "BB", which is long enough. In the process, (*MARK:2) is  encoun-
       tered  and  remembered.  When  the match attempt fails, the next "B" is
       found, but there is only one character left, so there are no  more  at-
       tempts,  and  "no  match"  is returned with the "last mark seen" set to
       "2". If NO_START_OPTIMIZE is set, however, matches are tried  at  every
       possible  starting position, including at the end of the subject, where
       (*MARK:1) is encountered, but there is no "B", so the "last mark  seen"
       that  is returned is "1". In this case, the optimizations do not affect
       the overall match result, which is still "no match", but they do affect
       the auxiliary information that is returned.

         PCRE2_NO_UTF_CHECK

       When PCRE2_UTF is set, the validity of the pattern as a UTF  string  is
       automatically  checked.  There  are  discussions  about the validity of
       UTF-8 strings, UTF-16 strings, and UTF-32 strings in  the  pcre2unicode
       document.  If an invalid UTF sequence is found, pcre2_compile() returns
       a negative error code.

       If you know that your pattern is a valid UTF string, and  you  want  to
       skip   this   check   for   performance   reasons,   you  can  set  the
       PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an in-
       valid UTF string as a pattern is undefined. It may cause  your  program
       to crash or loop.

       Note  that  this  option  can  also  be  passed  to  pcre2_match()  and
       pcre2_dfa_match(), to suppress UTF validity  checking  of  the  subject
       string.

       Note also that setting PCRE2_NO_UTF_CHECK at compile time does not dis-
       able  the error that is given if an escape sequence for an invalid Uni-
       code code point is encountered in the pattern. In particular,  the  so-
       called  "surrogate"  code points (0xd800 to 0xdfff) are invalid. If you
       want to allow escape  sequences  such  as  \x{d800}  you  can  set  the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES  extra  option, as described in the
       section entitled "Extra compile options" below.  However, this is  pos-
       sible only in UTF-8 and UTF-32 modes, because these values are not rep-
       resentable in UTF-16.

         PCRE2_UCP

       This option has two effects. Firstly, it change the way PCRE2 processes
       \B,  \b,  \D,  \d,  \S,  \s,  \W,  \w,  and some of the POSIX character
       classes. By default, only  ASCII  characters  are  recognized,  but  if
       PCRE2_UCP  is  set, Unicode properties are used to classify characters.
       There are some PCRE2_EXTRA options (see below) that add  finer  control
       to  this  behaviour.  More  details are given in the section on generic
       character types in the pcre2pattern page.

       The second effect of PCRE2_UCP is to force the use of  Unicode  proper-
       ties for upper/lower casing operations, even when PCRE2_UTF is not set.
       This  makes  it  possible  to process strings in the 16-bit UCS-2 code.
       This option is available only if PCRE2 has been compiled  with  Unicode
       support  (which is the default).  The PCRE2_EXTRA_CASELESS_RESTRICT op-
       tion (see below) restricts caseless matching such that ASCII characters
       match only ASCII characters and non-ASCII characters  match  only  non-
       ASCII characters.

         PCRE2_UNGREEDY

       This  option  inverts  the "greediness" of the quantifiers so that they
       are not greedy by default, but become greedy if followed by "?". It  is
       not  compatible  with Perl. It can also be set by a (?U) option setting
       within the pattern.

         PCRE2_USE_OFFSET_LIMIT

       This option must be set for pcre2_compile() if pcre2_set_offset_limit()
       is going to be used to set a non-default offset limit in a  match  con-
       text  for  matches  that  use this pattern. An error is generated if an
       offset limit is set without this option. For more details, see the  de-
       scription  of  pcre2_set_offset_limit()  in  the section that describes
       match contexts. See also the PCRE2_FIRSTLINE option above.

         PCRE2_UTF

       This option causes PCRE2 to regard both the  pattern  and  the  subject
       strings  that  are  subsequently processed as strings of UTF characters
       instead of single-code-unit strings. It  is  available  when  PCRE2  is
       built  to  include  Unicode  support (which is the default). If Unicode
       support is not available, the use of this option provokes an error. De-
       tails of how PCRE2_UTF changes the behaviour of PCRE2 are given in  the
       pcre2unicode  page.  In  particular,  note  that  it  changes  the  way
       PCRE2_CASELESS works.

   Extra compile options

       The option bits that can be set in a compile  context  by  calling  the
       pcre2_set_compile_extra_options() function are as follows:

         PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK

       Since release 10.38 PCRE2 has forbidden the use of \K within lookaround
       assertions, following Perl's lead. This option is provided to re-enable
       the previous behaviour (act in positive lookarounds, ignore in negative
       ones) in case anybody is relying on it.

         PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES

       This  option  applies when compiling a pattern in UTF-8 or UTF-32 mode.
       It is forbidden in UTF-16 mode, and ignored in non-UTF  modes.  Unicode
       "surrogate" code points in the range 0xd800 to 0xdfff are used in pairs
       in  UTF-16  to  encode  code points with values in the range 0x10000 to
       0x10ffff. The surrogates cannot therefore  be  represented  in  UTF-16.
       They can be represented in UTF-8 and UTF-32, but are defined as invalid
       code  points,  and  cause  errors  if  encountered in a UTF-8 or UTF-32
       string that is being checked for validity by PCRE2.

       These values also cause errors if encountered in escape sequences  such
       as \x{d912} within a pattern. However, it seems that some applications,
       when using PCRE2 to check for unwanted characters in UTF-8 strings, ex-
       plicitly   test   for   the  surrogates  using  escape  sequences.  The
       PCRE2_NO_UTF_CHECK option does not disable the error that  occurs,  be-
       cause it applies only to the testing of input strings for UTF validity.

       If  the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surro-
       gate code point values in UTF-8 and UTF-32 patterns no  longer  provoke
       errors  and are incorporated in the compiled pattern. However, they can
       only match subject characters if the matching function is  called  with
       PCRE2_NO_UTF_CHECK set.

         PCRE2_EXTRA_ALT_BSUX

       The  original option PCRE2_ALT_BSUX causes PCRE2 to process \U, \u, and
       \x in the way that ECMAscript (aka JavaScript) does.  Additional  func-
       tionality was defined by ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has
       the  effect  of PCRE2_ALT_BSUX, but in addition it recognizes \u{hhh..}
       as a hexadecimal character code, where hhh.. is any number of hexadeci-
       mal digits.

         PCRE2_EXTRA_ASCII_BSD

       This option forces \d to match only ASCII digits, even  when  PCRE2_UCP
       is  set.   It can be changed within a pattern by means of the (?aD) op-
       tion setting.

         PCRE2_EXTRA_ASCII_BSS

       This option forces \s to match only ASCII space characters,  even  when
       PCRE2_UCP  is  set.  It can be changed within a pattern by means of the
       (?aS) option setting.

         PCRE2_EXTRA_ASCII_BSW

       This option forces \w to match only ASCII word  characters,  even  when
       PCRE2_UCP  is  set.  It can be changed within a pattern by means of the
       (?aW) option setting.

         PCRE2_EXTRA_ASCII_DIGIT

       This option forces the POSIX character classes [:digit:] and [:xdigit:]
       to match only ASCII digits, even when  PCRE2_UCP  is  set.  It  can  be
       changed within a pattern by means of the (?aT) option setting.

         PCRE2_EXTRA_ASCII_POSIX

       This option forces all the POSIX character classes, including [:digit:]
       and  [:xdigit:], to match only ASCII characters, even when PCRE2_UCP is
       set. It can be changed within a pattern by means of  the  (?aP)  option
       setting,  but note that this also sets PCRE2_EXTRA_ASCII_DIGIT in order
       to ensure that (?-aP) unsets all ASCII restrictions for POSIX classes.

         PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL

       This is a dangerous option. Use with care. By default, an  unrecognized
       escape  such  as \j or a malformed one such as \x{2z} causes a compile-
       time error when detected by pcre2_compile(). Perl is somewhat inconsis-
       tent in handling such items: for example, \j is treated  as  a  literal
       "j",  and non-hexadecimal digits in \x{} are just ignored, though warn-
       ings are given in both cases if Perl's warning switch is enabled.  How-
       ever,  a  malformed  octal  number  after \o{ always causes an error in
       Perl.

       If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL  extra  option  is  passed  to
       pcre2_compile(),  all  unrecognized  or  malformed escape sequences are
       treated as single-character escapes. For example, \j is a  literal  "j"
       and  \x{2z}  is treated as the literal string "x{2z}". Setting this op-
       tion means that typos in patterns may go undetected and have unexpected
       results. Also note that a sequence such as [\N{] is  interpreted  as  a
       malformed  attempt  at [\N{...}] and so is treated as [N{] whereas [\N]
       gives an error because an unqualified \N is a valid escape sequence but
       is not supported in a character class. To reiterate: this is a  danger-
       ous option. Use with great care.

         PCRE2_EXTRA_CASELESS_RESTRICT

       When  either  PCRE2_UCP  or PCRE2_UTF is set, caseless matching follows
       Unicode rules, which allow for more than two cases per character. There
       are two case-equivalent character sets that contain both ASCII and non-
       ASCII characters. The ASCII letter S is case-equivalent to U+017f (long
       S) and the ASCII letter K is case-equivalent to U+212a  (Kelvin  sign).
       This  option  disables  recognition of case-equivalences that cross the
       ASCII/non-ASCII boundary. In a caseless match, both characters must ei-
       ther be ASCII or non-ASCII. The option can be changed with a pattern by
       the (?r) option setting.

         PCRE2_EXTRA_ESCAPED_CR_IS_LF

       There are some legacy applications where the escape sequence  \r  in  a
       pattern  is expected to match a newline. If this option is set, \r in a
       pattern is converted to \n so that it matches a LF  (linefeed)  instead
       of  a CR (carriage return) character. The option does not affect a lit-
       eral CR in the pattern, nor does it affect CR specified as an  explicit
       code point such as \x{0D}.

         PCRE2_EXTRA_MATCH_LINE

       This  option  is  provided  for  use  by the -x option of pcre2grep. It
       causes the pattern only to match complete lines. This  is  achieved  by
       automatically  inserting  the  code for "^(?:" at the start of the com-
       piled pattern and ")$" at the end. Thus, when PCRE2_MULTILINE  is  set,
       the  matched  line may be in the middle of the subject string. This op-
       tion can be used with PCRE2_LITERAL.

         PCRE2_EXTRA_MATCH_WORD

       This option is provided for use by  the  -w  option  of  pcre2grep.  It
       causes  the  pattern only to match strings that have a word boundary at
       the start and the end. This is achieved by automatically inserting  the
       code  for "\b(?:" at the start of the compiled pattern and ")\b" at the
       end. The option may be used with PCRE2_LITERAL. However, it is  ignored
       if PCRE2_EXTRA_MATCH_LINE is also set.


JUST-IN-TIME (JIT) COMPILATION

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize,
         size_t maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);

       These  functions  provide  support  for  JIT compilation, which, if the
       just-in-time compiler is available, further processes a  compiled  pat-
       tern into machine code that executes much faster than the pcre2_match()
       interpretive  matching function. Full details are given in the pcre2jit
       documentation.

       JIT compilation is a heavyweight optimization. It can  take  some  time
       for  patterns  to  be analyzed, and for one-off matches and simple pat-
       terns the benefit of faster execution might be offset by a much  slower
       compilation  time.  Most (but not all) patterns can be optimized by the
       JIT compiler.


LOCALE SUPPORT

       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
         const uint8_t *tables);

       PCRE2 handles caseless matching, and determines whether characters  are
       letters,  digits, or whatever, by reference to a set of tables, indexed
       by character code point. However, this applies only to characters whose
       code points are less than 256. By default,  higher-valued  code  points
       never match escapes such as \w or \d.

       When PCRE2 is built with Unicode support (the default), certain Unicode
       character  properties  can be tested with \p and \P, or, alternatively,
       the PCRE2_UCP option can be set when a pattern is compiled; this causes
       \w and friends to use Unicode property support instead of the  built-in
       tables.  PCRE2_UCP also causes upper/lower casing operations on charac-
       ters with code points greater than 127 to use Unicode properties. These
       effects  apply even when PCRE2_UTF is not set. There are, however, some
       PCRE2_EXTRA options (see above) that can be used to modify or  suppress
       them.

       The  use  of  locales  with Unicode is discouraged. If you are handling
       characters with code points greater than 127,  you  should  either  use
       Unicode support, or use locales, but not try to mix the two.

       PCRE2  contains a built-in set of character tables that are used by de-
       fault.  These are sufficient for many applications. Normally,  the  in-
       ternal  tables  recognize only ASCII characters. However, when PCRE2 is
       built, it is possible to cause the internal tables to be rebuilt in the
       default "C" locale of the local system, which may cause them to be dif-
       ferent.

       The built-in tables can be overridden by tables supplied by the  appli-
       cation  that  calls  PCRE2.  These may be created in a different locale
       from the default.  As more and more applications change to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are built by calling the pcre2_maketables() function,
       in the relevant locale. The only argument to this function is a general
       context, which can be used to pass a custom memory  allocator.  If  the
       argument is NULL, the system malloc() is used. The result can be passed
       to pcre2_compile() as often as necessary, by creating a compile context
       and  calling  pcre2_set_character_tables()  to  set  the tables pointer
       therein.

       For example, to build and use  tables  that  are  appropriate  for  the
       French  locale  (where accented characters with values greater than 127
       are treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre2_maketables(NULL);
         ccontext = pcre2_compile_context_create(NULL);
         pcre2_set_character_tables(ccontext, tables);
         re = pcre2_compile(..., ccontext);

       The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
       if you are using Windows, the name for the French locale is "french".

       The pointer that is passed (via the compile context) to pcre2_compile()
       is saved with the compiled pattern, and the same tables are used by the
       matching  functions.  Thus,  for  any  single  pattern, compilation and
       matching both happen in the same locale, but different patterns can  be
       processed in different locales.

       It  is the caller's responsibility to ensure that the memory containing
       the tables remains available while they are still in use. When they are
       no longer needed, you can discard them  using  pcre2_maketables_free(),
       which  should  pass as its first parameter the same global context that
       was used to create the tables.

   Saving locale tables

       The tables described above are just a sequence of binary  bytes,  which
       makes  them  independent of hardware characteristics such as endianness
       or whether the processor is 32-bit or 64-bit. A copy of the  result  of
       pcre2_maketables()  can  therefore  be saved in a file or elsewhere and
       re-used later, even in a different program or on another computer.  The
       size  of  the  tables  (number  of  bytes)  must be obtained by calling
       pcre2_config()  with  the  PCRE2_CONFIG_TABLES_LENGTH  option   because
       pcre2_maketables()   does   not   return  this  value.  Note  that  the
       pcre2_dftables program, which is part of the PCRE2 build system, can be
       used stand-alone to create a file that contains a set of binary tables.
       See the pcre2build documentation for details.


INFORMATION ABOUT A COMPILED PATTERN

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       The pcre2_pattern_info() function returns general information  about  a
       compiled pattern. For information about callouts, see the next section.
       The  first  argument  for pcre2_pattern_info() is a pointer to the com-
       piled pattern. The second argument specifies which piece of information
       is required, and the third argument is a pointer to a variable  to  re-
       ceive  the  data.  If the third argument is NULL, the first argument is
       ignored, and the function returns the size in  bytes  of  the  variable
       that is required for the information requested. Otherwise, the yield of
       the function is zero for success, or one of the following negative num-
       bers:

         PCRE2_ERROR_NULL           the argument code was NULL
         PCRE2_ERROR_BADMAGIC       the "magic number" was not found
         PCRE2_ERROR_BADOPTION      the value of what was invalid
         PCRE2_ERROR_UNSET          the requested field is not set

       The "magic number" is placed at the start of each compiled pattern as a
       simple  check  against  passing  an arbitrary memory pointer. Here is a
       typical call of pcre2_pattern_info(), to obtain the length of the  com-
       piled pattern:

         int rc;
         size_t length;
         rc = pcre2_pattern_info(
           re,               /* result of pcre2_compile() */
           PCRE2_INFO_SIZE,  /* what is required */
           &length);         /* where to put the data */

       The possible values for the second argument are defined in pcre2.h, and
       are as follows:

         PCRE2_INFO_ALLOPTIONS
         PCRE2_INFO_ARGOPTIONS
         PCRE2_INFO_EXTRAOPTIONS

       Return copies of the pattern's options. The third argument should point
       to  a  uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the op-
       tions that were passed to  pcre2_compile(),  whereas  PCRE2_INFO_ALLOP-
       TIONS  returns  the compile options as modified by any top-level (*XXX)
       option settings such as (*UTF) at the  start  of  the  pattern  itself.
       PCRE2_INFO_EXTRAOPTIONS  returns the extra options that were set in the
       compile context by calling the pcre2_set_compile_extra_options()  func-
       tion.

       For  example, if the pattern /(*UTF)abc/ is compiled with the PCRE2_EX-
       TENDED option, the result for PCRE2_INFO_ALLOPTIONS  is  PCRE2_EXTENDED
       and  PCRE2_UTF.   Option settings such as (?i) that can change within a
       pattern do not affect the result of PCRE2_INFO_ALLOPTIONS, even if they
       appear right at the start of the pattern. (This was different  in  some
       earlier releases.)

       A  pattern compiled without PCRE2_ANCHORED is automatically anchored by
       PCRE2 if the first significant item in every top-level branch is one of
       the following:

         ^     unless PCRE2_MULTILINE is set
         \A    always
         \G    always
         .*    sometimes - see below

       When .* is the first significant item, anchoring is possible only  when
       all the following are true:

         .* is not in an atomic group
         .* is not in a capture group that is the subject
              of a backreference
         PCRE2_DOTALL is in force for .*
         Neither (*PRUNE) nor (*SKIP) appears in the pattern
         PCRE2_NO_DOTSTAR_ANCHOR is not set

       For  patterns  that are auto-anchored, the PCRE2_ANCHORED bit is set in
       the options returned for PCRE2_INFO_ALLOPTIONS.

         PCRE2_INFO_BACKREFMAX

       Return the number of the highest  backreference  in  the  pattern.  The
       third  argument  should  point  to  a  uint32_t variable. Named capture
       groups acquire numbers as well as names, and these  count  towards  the
       highest  backreference.  Backreferences  such as \4 or \g{12} match the
       captured characters of the given group, but in addition, the check that
       a capture group is set in a conditional group such as (?(3)a|b) is also
       a backreference.  Zero is returned if there are no backreferences.

         PCRE2_INFO_BSR

       The output is a uint32_t integer whose value indicates  what  character
       sequences  the \R escape sequence matches. A value of PCRE2_BSR_UNICODE
       means that \R matches any Unicode line  ending  sequence;  a  value  of
       PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF.

         PCRE2_INFO_CAPTURECOUNT

       Return  the  highest  capture  group number in the pattern. In patterns
       where (?| is not used, this is also the total number of capture groups.
       The third argument should point to a uint32_t variable.

         PCRE2_INFO_DEPTHLIMIT

       If the pattern set a backtracking depth limit by including an  item  of
       the  form  (*LIMIT_DEPTH=nnnn) at the start, the value is returned. The
       third argument should point to a uint32_t integer. If no such value has
       been set, the call to pcre2_pattern_info() returns the error  PCRE2_ER-
       ROR_UNSET. Note that this limit will only be used during matching if it
       is  less  than  the  limit  set or defaulted by the caller of the match
       function.

         PCRE2_INFO_FIRSTBITMAP

       In the absence of a single first code unit for a non-anchored  pattern,
       pcre2_compile()  may construct a 256-bit table that defines a fixed set
       of values for the first code unit in any match. For example, a  pattern
       that  starts  with  [abc]  results in a table with three bits set. When
       code unit values greater than 255 are supported, the flag bit  for  255
       means  "any  code unit of value 255 or above". If such a table was con-
       structed, a pointer to it is returned. Otherwise NULL is returned.  The
       third argument should point to a const uint8_t * variable.

         PCRE2_INFO_FIRSTCODETYPE

       Return information about the first code unit of any matched string, for
       a  non-anchored  pattern. The third argument should point to a uint32_t
       variable. If there is a fixed first value, for example, the letter  "c"
       from  a  pattern such as (cat|cow|coyote), 1 is returned, and the value
       can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is  no  fixed
       first  value,  but it is known that a match can occur only at the start
       of the subject or following a newline in the subject,  2  is  returned.
       Otherwise, and for anchored patterns, 0 is returned.

         PCRE2_INFO_FIRSTCODEUNIT

       Return  the  value  of  the first code unit of any matched string for a
       pattern where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise  return  0.
       The  third  argument  should point to a uint32_t variable. In the 8-bit
       library, the value is always less than 256. In the 16-bit  library  the
       value  can  be  up  to 0xffff. In the 32-bit library in UTF-32 mode the
       value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32
       mode.

         PCRE2_INFO_FRAMESIZE

       Return the size (in bytes) of the data frames that are used to remember
       backtracking positions when the pattern is processed  by  pcre2_match()
       without  the  use  of  JIT. The third argument should point to a size_t
       variable. The frame size depends on the number of capturing parentheses
       in the pattern. Each additional capture group adds two PCRE2_SIZE vari-
       ables.

         PCRE2_INFO_HASBACKSLASHC

       Return 1 if the pattern contains any instances of \C, otherwise 0.  The
       third argument should point to a uint32_t variable.

         PCRE2_INFO_HASCRORLF

       Return  1  if  the  pattern  contains any explicit matches for CR or LF
       characters, otherwise 0. The third argument should point to a  uint32_t
       variable.  An explicit match is either a literal CR or LF character, or
       \r or \n or one of the  equivalent  hexadecimal  or  octal  escape  se-
       quences.

         PCRE2_INFO_HEAPLIMIT

       If the pattern set a heap memory limit by including an item of the form
       (*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argu-
       ment should point to a uint32_t integer. If no such value has been set,
       the  call  to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET.
       Note that this limit will only be used during matching if  it  is  less
       than the limit set or defaulted by the caller of the match function.

         PCRE2_INFO_JCHANGED

       Return  1  if  the (?J) or (?-J) option setting is used in the pattern,
       otherwise 0. The third argument should point to  a  uint32_t  variable.
       (?J)  and  (?-J) set and unset the local PCRE2_DUPNAMES option, respec-
       tively.

         PCRE2_INFO_JITSIZE

       If the compiled pattern was successfully  processed  by  pcre2_jit_com-
       pile(),  return  the  size  of  the JIT compiled code, otherwise return
       zero. The third argument should point to a size_t variable.

         PCRE2_INFO_LASTCODETYPE

       Returns 1 if there is a rightmost literal code unit that must exist  in
       any  matched string, other than at its start. The third argument should
       point to a uint32_t variable. If there is no such value, 0 is returned.
       When 1 is returned, the code unit value itself can be  retrieved  using
       PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is
       recorded  only if it follows something of variable length. For example,
       for the pattern /^a\d+z\d+/ the returned value is 1 (with "z"  returned
       from  PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is
       0.

         PCRE2_INFO_LASTCODEUNIT

       Return the value of the rightmost literal code unit that must exist  in
       any  matched  string,  other  than  at  its  start, for a pattern where
       PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argu-
       ment should point to a uint32_t variable.

         PCRE2_INFO_MATCHEMPTY

       Return 1 if the pattern might match an empty string, otherwise  0.  The
       third argument should point to a uint32_t variable. When a pattern con-
       tains recursive subroutine calls it is not always possible to determine
       whether or not it can match an empty string. PCRE2 takes a cautious ap-
       proach and returns 1 in such cases.

         PCRE2_INFO_MATCHLIMIT

       If  the  pattern  set  a  match  limit by including an item of the form
       (*LIMIT_MATCH=nnnn) at the start, the value is returned. The third  ar-
       gument  should  point  to a uint32_t integer. If no such value has been
       set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UN-
       SET. Note that this limit will only be used during matching  if  it  is
       less  than  the limit set or defaulted by the caller of the match func-
       tion.

         PCRE2_INFO_MAXLOOKBEHIND

       A lookbehind assertion moves back a certain number of  characters  (not
       code  units)  when  it starts to process each of its branches. This re-
       quest returns the largest of these backward moves. The  third  argument
       should point to a uint32_t integer. The simple assertions \b and \B re-
       quire  a one-character lookbehind and cause PCRE2_INFO_MAXLOOKBEHIND to
       return 1 in the absence of anything longer. \A also  registers  a  one-
       character  lookbehind, though it does not actually inspect the previous
       character.

       Note that this information is useful for multi-segment matching only if
       the pattern contains no nested lookbehinds. For  example,  the  pattern
       (?<=a(?<=ba)c)  returns  a  maximum  lookbehind  of  2,  but when it is
       processed, the first lookbehind moves back by two  characters,  matches
       one  character, then the nested lookbehind also moves back by two char-
       acters. This puts the matching point three characters earlier  than  it
       was  at the start.  PCRE2_INFO_MAXLOOKBEHIND is really only useful as a
       debugging tool. See the pcre2partial documentation for a discussion  of
       multi-segment matching.

         PCRE2_INFO_MINLENGTH

       If  a  minimum  length  for  matching subject strings was computed, its
       value is returned. Otherwise the returned value is 0. This value is not
       computed when PCRE2_NO_START_OPTIMIZE is set. The value is a number  of
       characters,  which in UTF mode may be different from the number of code
       units. The third argument should point  to  a  uint32_t  variable.  The
       value  is a lower bound to the length of any matching string. There may
       not be any strings of that length that do  actually  match,  but  every
       string that does match is at least that long.

         PCRE2_INFO_NAMECOUNT
         PCRE2_INFO_NAMEENTRYSIZE
         PCRE2_INFO_NAMETABLE

       PCRE2 supports the use of named as well as numbered capturing parenthe-
       ses.  The names are just an additional way of identifying the parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre2_substring_get_byname() are provided for extracting captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers  in the output vector (described with pcre2_match() below). To
       do the conversion, you need to use the name-to-number map, which is de-
       scribed by these three values.

       The map consists of a number of  fixed-size  entries.  PCRE2_INFO_NAME-
       COUNT  gives  the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives
       the size of each entry in code units; both of these return  a  uint32_t
       value. The entry size depends on the length of the longest name.

       PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table.
       This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit li-
       brary,  the first two bytes of each entry are the number of the captur-
       ing parenthesis, most significant byte first. In  the  16-bit  library,
       the  pointer  points  to 16-bit code units, the first of which contains
       the parenthesis number. In the 32-bit library, the  pointer  points  to
       32-bit  code units, the first of which contains the parenthesis number.
       The rest of the entry is the corresponding name, zero terminated.

       The names are in alphabetical order. If (?| is used to create  multiple
       capture groups with the same number, as described in the section on du-
       plicate group numbers in the pcre2pattern page, the groups may be given
       the  same  name,  but  there  is only one entry in the table. Different
       names for groups of the same number are not permitted.

       Duplicate names for capture groups with different numbers  are  permit-
       ted, but only if PCRE2_DUPNAMES is set. They appear in the table in the
       order  in  which  they were found in the pattern. In the absence of (?|
       this is the order of increasing number; when (?| is used  this  is  not
       necessarily  the  case because later capture groups may have lower num-
       bers.

       As a simple example of the name/number table,  consider  the  following
       pattern  after  compilation by the 8-bit library (assume PCRE2_EXTENDED
       is set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named capture groups, so the table has four entries, and
       each entry in the table is eight bytes long. The table is  as  follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code to extract data from named capture groups using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE2_INFO_NEWLINE

       The output is one of the following uint32_t values:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       This identifies the character sequence that will be recognized as mean-
       ing "newline" while matching.

         PCRE2_INFO_SIZE

       Return  the  size  of  the compiled pattern in bytes (for all three li-
       braries). The third argument should point to a  size_t  variable.  This
       value  includes  the  size  of the general data block that precedes the
       code units of the compiled pattern itself. The value that is used  when
       pcre2_compile()  is  getting memory in which to place the compiled pat-
       tern may be slightly larger than the value returned by this option, be-
       cause there are cases where the code that calculates the  size  has  to
       over-estimate.  Processing a pattern with the JIT compiler does not al-
       ter the value returned by this option.


INFORMATION ABOUT A PATTERN'S CALLOUTS

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might like to scan all the callouts in a  pattern  before  running  the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument  is  a  pointer  to a compiled pattern, the second points to a
       callback function, and the third is arbitrary user data.  The  callback
       function  is  called  for  every callout in the pattern in the order in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation block, and its second argument is the user_data  value  that  was
       passed  to  pcre2_callout_enumerate(). The contents of the callout enu-
       meration block are described in the pcre2callout  documentation,  which
       also gives further details about callouts.


SERIALIZATION AND PRECOMPILING

       It  is possible to save compiled patterns on disc or elsewhere, and re-
       load them later, subject to a number of restrictions. The host on which
       the patterns are reloaded must be running the same  version  of  PCRE2,
       with  the same code unit width, and must also have the same endianness,
       pointer width, and PCRE2_SIZE type. Before  compiled  patterns  can  be
       saved, they must be converted to a "serialized" form, which in the case
       of PCRE2 is really just a bytecode dump.  The functions whose names be-
       gin with pcre2_serialize_ are used for converting to and from the seri-
       alized  form.  They  are described in the pcre2serialize documentation.
       Note that PCRE2 serialization does not convert compiled patterns to  an
       abstract format like Java or .NET serialization.


THE MATCH DATA BLOCK

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       void pcre2_match_data_free(pcre2_match_data *match_data);

       Information  about  a  successful  or unsuccessful match is placed in a
       match data block, which is an opaque  structure  that  is  accessed  by
       function  calls.  In particular, the match data block contains a vector
       of offsets into the subject string that define the matched parts of the
       subject. This is known as the ovector.

       Before calling pcre2_match(), pcre2_dfa_match(),  or  pcre2_jit_match()
       you must create a match data block by calling one of the creation func-
       tions  above.  For pcre2_match_data_create(), the first argument is the
       number of pairs of offsets in the ovector.

       When using pcre2_match(), one pair of offsets is required  to  identify
       the  string that matched the whole pattern, with an additional pair for
       each captured substring. For example, a value of 4 creates enough space
       to record the matched portion of the subject plus three  captured  sub-
       strings.

       When  using  pcre2_dfa_match() there may be multiple matched substrings
       of different lengths at the same point  in  the  subject.  The  ovector
       should be made large enough to hold as many as are expected.

       A  minimum  of at least 1 pair is imposed by pcre2_match_data_create(),
       so it is always possible to return the overall matched  string  in  the
       case   of   pcre2_match()   or   the  longest  match  in  the  case  of
       pcre2_dfa_match(). The maximum number of pairs is 65535; if  the  first
       argument  of  pcre2_match_data_create()  is greater than this, 65535 is
       used.

       The second argument of pcre2_match_data_create() is a pointer to a gen-
       eral context, which can specify custom memory management for  obtaining
       the memory for the match data block. If you are not using custom memory
       management, pass NULL, which causes malloc() to be used.

       For  pcre2_match_data_create_from_pattern(),  the  first  argument is a
       pointer to a compiled pattern. The ovector is created to be exactly the
       right size to hold all the substrings  a  pattern  might  capture  when
       matched using pcre2_match(). You should not use this call when matching
       with  pcre2_dfa_match().  The  second  argument is again a pointer to a
       general context, but in this case if NULL is passed, the memory is  ob-
       tained  using the same allocator that was used for the compiled pattern
       (custom or default).

       A match data block can be used many times, with the same  or  different
       compiled  patterns. You can extract information from a match data block
       after a match operation has finished,  using  functions  that  are  de-
       scribed in the sections on matched strings and other match data below.

       When  a  call  of  pcre2_match()  fails, valid data is available in the
       match block only  when  the  error  is  PCRE2_ERROR_NOMATCH,  PCRE2_ER-
       ROR_PARTIAL,  or  one of the error codes for an invalid UTF string. Ex-
       actly what is available depends on the error, and is detailed below.

       When one of the matching functions is called, pointers to the  compiled
       pattern  and the subject string are set in the match data block so that
       they can be referenced by the extraction functions after  a  successful
       match. After running a match, you must not free a compiled pattern or a
       subject  string until after all operations on the match data block (for
       that match) have taken place,  unless,  in  the  case  of  the  subject
       string,  you  have used the PCRE2_COPY_MATCHED_SUBJECT option, which is
       described in the section entitled "Option bits for  pcre2_match()"  be-
       low.

       When  a match data block itself is no longer needed, it should be freed
       by calling pcre2_match_data_free(). If this function is called  with  a
       NULL argument, it returns immediately, without doing anything.


MEMORY USE FOR MATCH DATA BLOCKS

       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
         pcre2_match_data *match_data);

       The  size of a match data block depends on the size of the ovector that
       it contains. The function pcre2_get_match_data_size() returns the size,
       in bytes, of the block that is its argument.

       When pcre2_match() runs interpretively (that is, without using JIT), it
       makes use of a vector of data frames for remembering backtracking posi-
       tions.  The size of each individual frame depends on the number of cap-
       turing parentheses in the  pattern  and  can  be  obtained  by  calling
       pcre2_pattern_info() with the PCRE2_INFO_FRAMESIZE option (see the sec-
       tion entitled "Information about a compiled pattern" above).

       Heap  memory is used for the frames vector; if the initial memory block
       turns out to be too small during  matching,  it  is  automatically  ex-
       panded.  When  pcre2_match()  returns, the memory is not freed, but re-
       mains attached to the match data  block,  for  use  by  any  subsequent
       matches  that  use  the  same block. It is automatically freed when the
       match data block itself is freed.

       You can find the current size of the frames vector that  a  match  data
       block  owns  by  calling  pcre2_get_match_data_heapframes_size(). For a
       newly created match data block the size will be  zero.  Some  types  of
       match may require a lot of frames and thus a large vector; applications
       that run in environments where memory is constrained can check this and
       free the match data block if the heap frames vector has become too big.


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       The  function pcre2_match() is called to match a subject string against
       a compiled pattern, which is passed in the code argument. You can  call
       pcre2_match() with the same code argument as many times as you like, in
       order  to  find multiple matches in the subject string or to match dif-
       ferent subject strings with the same pattern.

       This function is the main matching facility of the library, and it  op-
       erates  in  a Perl-like manner. For specialist use there is also an al-
       ternative matching function, which is described below  in  the  section
       about the pcre2_dfa_match() function.

       Here is an example of a simple call to pcre2_match():

         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL);          /* a match context; NULL means use defaults */

       If  the  subject  string is zero-terminated, the length can be given as
       PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
       common matching parameters are to be changed. For details, see the sec-
       tion on the match context above.

   The string to be matched by pcre2_match()

       The subject string is passed to pcre2_match() as a pointer in  subject,
       a  length  in  length, and a starting offset in startoffset. The length
       and offset are in code units, not characters.  That  is,  they  are  in
       bytes  for the 8-bit library, 16-bit code units for the 16-bit library,
       and 32-bit code units for the 32-bit library, whether or not  UTF  pro-
       cessing is enabled. As a special case, if subject is NULL and length is
       zero,  the  subject is assumed to be an empty string. If length is non-
       zero, an error occurs if subject is NULL.

       If startoffset is greater than the length of the subject, pcre2_match()
       returns PCRE2_ERROR_BADOFFSET. When the starting offset  is  zero,  the
       search  for a match starts at the beginning of the subject, and this is
       by far the most common case. In UTF-8 or UTF-16 mode, the starting off-
       set must point to the start of a character, or to the end of  the  sub-
       ject  (in  UTF-32 mode, one code unit equals one character, so all off-
       sets are valid). Like the pattern string, the subject may  contain  bi-
       nary zeros.

       A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre2_match()  again  after  a  previous
       success.   Setting  startoffset  differs  from passing over a shortened
       string and setting PCRE2_NOTBOL in the case of a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When   applied   to   the   string  "Mississippi"  the  first  call  to
       pcre2_match() finds the first occurrence. If  pcre2_match()  is  called
       again with just the remainder of the subject, namely "issippi", it does
       not  match,  because  \B  is  always false at the start of the subject,
       which is deemed to be a word boundary.  However,  if  pcre2_match()  is
       passed the entire string again, but with startoffset set to 4, it finds
       the  second  occurrence  of "iss" because it is able to look behind the
       starting point to discover that it is preceded by a letter.

       Finding all the matches in a subject is tricky  when  the  pattern  can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first   trying   the   match   again  at  the  same  offset,  with  the
       PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options,  and  then  if  that
       fails,  advancing  the  starting  offset  and  trying an ordinary match
       again. There is some code that demonstrates  how  to  do  this  in  the
       pcre2demo  sample  program. In the most general case, you have to check
       to see if the newline convention recognizes CRLF as a newline,  and  if
       so,  and the current character is CR followed by LF, advance the start-
       ing offset by two characters instead of one.

       If a non-zero starting offset is passed when the pattern is anchored, a
       single attempt to match at the given offset is made. This can only suc-
       ceed if the pattern does not require the match to be at  the  start  of
       the  subject.  In other words, the anchoring must be the result of set-
       ting the PCRE2_ANCHORED option or the use of .* with PCRE2_DOTALL,  not
       by starting the pattern with ^ or \A.

   Option bits for pcre2_match()

       The unused bits of the options argument for pcre2_match() must be zero.
       The    only    bits    that    may    be    set   are   PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_DISABLE_RECURSELOOP_CHECK,  PCRE2_EN-
       DANCHORED,       PCRE2_NOTBOL,       PCRE2_NOTEOL,      PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_JIT,  PCRE2_NO_UTF_CHECK,  PCRE2_PAR-
       TIAL_HARD, and PCRE2_PARTIAL_SOFT.  Their action is described below.

       Setting  PCRE2_ANCHORED  or PCRE2_ENDANCHORED at match time is not sup-
       ported by the just-in-time (JIT) compiler. If it is set,  JIT  matching
       is  disabled  and  the  interpretive  code  in  pcre2_match()  is  run.
       PCRE2_DISABLE_RECURSELOOP_CHECK is  ignored  by  JIT,  but  apart  from
       PCRE2_NO_JIT  (obviously),  the remaining options are supported for JIT
       matching.

         PCRE2_ANCHORED

       The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
       matching position. If a pattern was compiled  with  PCRE2_ANCHORED,  or
       turned  out to be anchored by virtue of its contents, it cannot be made
       unachored at matching time. Note that setting the option at match  time
       disables JIT matching.

         PCRE2_COPY_MATCHED_SUBJECT

       By  default,  a  pointer to the subject is remembered in the match data
       block so that, after a successful match, it can be  referenced  by  the
       substring  extraction  functions.  This means that the subject's memory
       must not be freed until all such operations are complete. For some  ap-
       plications  where the lifetime of the subject string is not guaranteed,
       it may be necessary to make a copy of the subject  string,  but  it  is
       wasteful  to do this unless the match is successful. After a successful
       match, if PCRE2_COPY_MATCHED_SUBJECT is set, the subject is copied  and
       the  new  pointer  is remembered in the match data block instead of the
       original subject pointer. The memory allocator that was  used  for  the
       match  block  itself  is  used.  The  copy  is automatically freed when
       pcre2_match_data_free() is called to free the match data block.  It  is
       also automatically freed if the match data block is re-used for another
       match operation.

         PCRE2_DISABLE_RECURSELOOP_CHECK

       This  option  is relevant only to pcre2_match() for interpretive match-
       ing.   It  is  ignored  when  JIT  is  used,  and  is   forbidden   for
       pcre2_dfa_match().

       The use of recursion in patterns can lead to infinite loops. In the in-
       terpretive  matcher  these  would  be eventually caught by the match or
       heap limits, but this could take a long time and/or use a lot of memory
       if the limits are large. There is therefore a check  at  the  start  of
       each  recursion.   If  the  same  group is still active from a previous
       call, and the current subject pointer is the same  as  it  was  at  the
       start  of  that group, and the furthest inspected character of the sub-
       ject has not changed, an error is generated.

       There are rare cases of matches that would complete,  but  nevertheless
       trigger  this  error.  This  option  disables the check. It is provided
       mainly for testing when comparing JIT and interpretive behaviour.

         PCRE2_ENDANCHORED

       If the PCRE2_ENDANCHORED option is set, any string  that  pcre2_match()
       matches  must be right at the end of the subject string. Note that set-
       ting the option at match time disables JIT matching.

         PCRE2_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before  it.  Setting  this without having set PCRE2_MULTILINE at
       compile time causes circumflex never to match. This option affects only
       the behaviour of the circumflex metacharacter. It does not affect \A.

         PCRE2_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out having set PCRE2_MULTILINE at compile time causes dollar  never  to
       match. This option affects only the behaviour of the dollar metacharac-
       ter. It does not affect \Z or \z.

         PCRE2_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
       match is not valid, so pcre2_match() searches further into  the  string
       for occurrences of "a" or "b".

         PCRE2_NOTEMPTY_ATSTART

       This  is  like PCRE2_NOTEMPTY, except that it locks out an empty string
       match only at the first matching position, that is, at the start of the
       subject plus the starting offset. An empty string match  later  in  the
       subject is permitted.  If the pattern is anchored, such a match can oc-
       cur only if the pattern contains \K.

         PCRE2_NO_JIT

       By   default,   if   a  pattern  has  been  successfully  processed  by
       pcre2_jit_compile(), JIT is automatically used  when  pcre2_match()  is
       called  with  options  that JIT supports. Setting PCRE2_NO_JIT disables
       the use of JIT; it forces matching to be done by the interpreter.

         PCRE2_NO_UTF_CHECK

       When PCRE2_UTF is set at compile time, the validity of the subject as a
       UTF  string  is  checked  unless  PCRE2_NO_UTF_CHECK   is   passed   to
       pcre2_match() or PCRE2_MATCH_INVALID_UTF was passed to pcre2_compile().
       The latter special case is discussed in detail in the pcre2unicode doc-
       umentation.

       In  the default case, if a non-zero starting offset is given, the check
       is applied only to that part of the subject  that  could  be  inspected
       during  matching,  and there is a check that the starting offset points
       to the first code unit of a character or to the end of the subject.  If
       there  are no lookbehind assertions in the pattern, the check starts at
       the starting offset.  Otherwise, it starts at the length of the longest
       lookbehind before the starting offset, or at the start of  the  subject
       if  there are not that many characters before the starting offset. Note
       that the sequences \b and \B are one-character lookbehinds.

       The check is carried out before any other processing takes place, and a
       negative error code is returned if the check fails. There  are  several
       UTF  error  codes  for each code unit width, corresponding to different
       problems with the code unit sequence. There are discussions  about  the
       validity  of  UTF-8  strings, UTF-16 strings, and UTF-32 strings in the
       pcre2unicode documentation.

       If you know that your subject is valid, and you want to skip this check
       for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when
       calling pcre2_match(). You might want to do this  for  the  second  and
       subsequent  calls  to pcre2_match() if you are making repeated calls to
       find multiple matches in the same subject string.

       Warning: Unless PCRE2_MATCH_INVALID_UTF was set at compile  time,  when
       PCRE2_NO_UTF_CHECK  is  set  at match time the effect of passing an in-
       valid string as a subject, or an invalid value of startoffset, is unde-
       fined.  Your program may crash or loop indefinitely or give  wrong  re-
       sults.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These options turn on the partial matching feature. A partial match oc-
       curs  if  the  end  of  the subject string is reached successfully, but
       there are not enough subject characters to complete the match. In addi-
       tion, either at least one character must have  been  inspected  or  the
       pattern  must  contain  a  lookbehind,  or the pattern must be one that
       could match an empty string.

       If this situation arises when PCRE2_PARTIAL_SOFT  (but  not  PCRE2_PAR-
       TIAL_HARD) is set, matching continues by testing any remaining alterna-
       tives.  Only  if  no complete match can be found is PCRE2_ERROR_PARTIAL
       returned instead of PCRE2_ERROR_NOMATCH.  In  other  words,  PCRE2_PAR-
       TIAL_SOFT  specifies  that  the  caller is prepared to handle a partial
       match, but only if no complete match can be found.

       If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In  this
       case,  if  a  partial match is found, pcre2_match() immediately returns
       PCRE2_ERROR_PARTIAL, without considering  any  other  alternatives.  In
       other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important that an alternative complete match.

       There is a more detailed discussion of partial and multi-segment match-
       ing, with examples, in the pcre2partial documentation.


NEWLINE HANDLING WHEN MATCHING

       When  PCRE2 is built, a default newline convention is set; this is usu-
       ally the standard convention for the operating system. The default  can
       be  overridden  in a compile context by calling pcre2_set_newline(). It
       can also be overridden by starting a pattern string with, for  example,
       (*CRLF),  as  described  in  the  section on newline conventions in the
       pcre2pattern page. During matching, the newline choice affects the  be-
       haviour  of the dot, circumflex, and dollar metacharacters. It may also
       alter the way the match starting position is  advanced  after  a  match
       failure for an unanchored pattern.

       When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
       set  as  the  newline convention, and a match attempt for an unanchored
       pattern fails when the current starting position is at a CRLF sequence,
       and the pattern contains no explicit matches for CR or  LF  characters,
       the  match  position  is  advanced by two characters instead of one, in
       other words, to after the CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected. For example, if the pattern is .+A (and the PCRE2_DOTALL  op-
       tion  is  not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before  retrying.
       However,  the  pattern  [\r\n]A does match that string, because it con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those characters in the pattern, or one of the \r or \n  or  equivalent
       octal or hexadecimal escape sequences. Implicit matches such as [^X] do
       not  count, nor does \s, even though it includes CR and LF in the char-
       acters that it matches.

       Notwithstanding the above, anomalous effects may still occur when  CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.


HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parenthesized  parts  of  the  pattern.  Following the usage in Jeffrey
       Friedl's book, this is called "capturing"  in  what  follows,  and  the
       phrase  "capture  group" (Perl terminology) is used for a fragment of a
       pattern that picks out a substring. PCRE2 supports several other  kinds
       of parenthesized group that do not cause substrings to be captured. The
       pcre2_pattern_info()  function can be used to find out how many capture
       groups there are in a compiled pattern.

       You can use auxiliary functions for accessing  captured  substrings  by
       number or by name, as described in sections below.

       Alternatively, you can make direct use of the vector of PCRE2_SIZE val-
       ues,  called  the  ovector,  which  contains  the  offsets  of captured
       strings.  It  is  part  of  the  match  data   block.    The   function
       pcre2_get_ovector_pointer()  returns  the  address  of the ovector, and
       pcre2_get_ovector_count() returns the number of pairs of values it con-
       tains.

       Within the ovector, the first in each pair of values is set to the off-
       set of the first code unit of a substring, and the second is set to the
       offset of the first code unit after the end of a substring. These  val-
       ues  are always code unit offsets, not character offsets. That is, they
       are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit li-
       brary, and 32-bit offsets in the 32-bit library.

       After a partial match  (error  return  PCRE2_ERROR_PARTIAL),  only  the
       first  pair  of  offsets  (that is, ovector[0] and ovector[1]) are set.
       They identify the part of the subject that was partially  matched.  See
       the pcre2partial documentation for details of partial matching.

       After  a  fully  successful match, the first pair of offsets identifies
       the portion of the subject string that was matched by the  entire  pat-
       tern.  The  next  pair is used for the first captured substring, and so
       on. The value returned by pcre2_match() is one more  than  the  highest
       numbered  pair  that  has been set. For example, if two substrings have
       been captured, the returned value is 3. If there are no  captured  sub-
       strings, the return value from a successful match is 1, indicating that
       just the first pair of offsets has been set.

       If  a  pattern uses the \K escape sequence within a positive assertion,
       the reported start of a successful match can be greater than the end of
       the match.  For example, if the pattern  (?=ab\K)  is  matched  against
       "ab", the start and end offset values for the match are 2 and 0.

       If  a  capture group is matched repeatedly within a single match opera-
       tion, it is the last portion of the subject that it matched that is re-
       turned.

       If the ovector is too small to hold all the captured substring offsets,
       as much as possible is filled in, and the function returns a  value  of
       zero.  If captured substrings are not of interest, pcre2_match() may be
       called with a match data block whose ovector is of minimum length (that
       is, one pair).

       It is possible for capture group number n+1 to match some part  of  the
       subject  when  group  n  has  not been used at all. For example, if the
       string "abc" is matched against the pattern (a|(z))(bc) the return from
       the function is 4, and groups 1 and 3 are matched, but 2 is  not.  When
       this  happens,  both values in the offset pairs corresponding to unused
       groups are set to PCRE2_UNSET.

       Offset values that correspond to unused groups at the end  of  the  ex-
       pression  are also set to PCRE2_UNSET. For example, if the string "abc"
       is matched against the pattern (abc)(x(yz)?)? groups 2 and  3  are  not
       matched.  The  return  from the function is 2, because the highest used
       capture group number is 1. The offsets for the second and third capture
       groups (assuming the vector is large enough,  of  course)  are  set  to
       PCRE2_UNSET.

       Elements in the ovector that do not correspond to capturing parentheses
       in the pattern are never changed. That is, if a pattern contains n cap-
       turing parentheses, no more than ovector[0] to ovector[2n+1] are set by
       pcre2_match().  The  other  elements retain whatever values they previ-
       ously had. After a failed match attempt, the contents  of  the  ovector
       are unchanged.


OTHER INFORMATION ABOUT A MATCH

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);

       As  well as the offsets in the ovector, other information about a match
       is retained in the match data block and can be retrieved by  the  above
       functions  in  appropriate  circumstances.  If they are called at other
       times, the result is undefined.

       After a successful match, a partial match (PCRE2_ERROR_PARTIAL),  or  a
       failure  to  match (PCRE2_ERROR_NOMATCH), a mark name may be available.
       The function pcre2_get_mark() can be called to access this name,  which
       can  be  specified  in  the  pattern by any of the backtracking control
       verbs, not just (*MARK). The same function applies to all the verbs. It
       returns a pointer to the zero-terminated name, which is within the com-
       piled pattern. If no name is available, NULL is returned. The length of
       the name (excluding the terminating zero) is stored in  the  code  unit
       that  precedes  the name. You should use this length instead of relying
       on the terminating zero if the name might contain a binary zero.

       After a successful match, the name that is returned is  the  last  mark
       name encountered on the matching path through the pattern. Instances of
       backtracking  verbs  without  names do not count. Thus, for example, if
       the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned.
       After a "no match" or a partial match, the last encountered name is re-
       turned. For example, consider this pattern:

         ^(*MARK:A)((*MARK:B)a|b)c

       When it matches "bc", the returned name is A. The B mark is  "seen"  in
       the  first  branch of the group, but it is not on the matching path. On
       the other hand, when this pattern fails to  match  "bx",  the  returned
       name is B.

       Warning:  By  default, certain start-of-match optimizations are used to
       give a fast "no match" result in some situations. For example,  if  the
       anchoring  is removed from the pattern above, there is an initial check
       for the presence of "c" in the subject before running the matching  en-
       gine. This check fails for "bx", causing a match failure without seeing
       any  marks. You can disable the start-of-match optimizations by setting
       the PCRE2_NO_START_OPTIMIZE option for pcre2_compile() or  by  starting
       the pattern with (*NO_START_OPT).

       After  a  successful  match, a partial match, or one of the invalid UTF
       errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar()  can
       be called. After a successful or partial match it returns the code unit
       offset  of  the character at which the match started. For a non-partial
       match, this can be different to the value of ovector[0] if the  pattern
       contains  the  \K escape sequence. After a partial match, however, this
       value is always the same as ovector[0] because \K does not  affect  the
       result of a partial match.

       After  a UTF check failure, pcre2_get_startchar() can be used to obtain
       the code unit offset of the invalid UTF character. Details are given in
       the pcre2unicode page.


ERROR RETURNS FROM pcre2_match()

       If pcre2_match() fails, it returns a negative number. This can be  con-
       verted  to a text string by calling the pcre2_get_error_message() func-
       tion (see "Obtaining a textual error message" below).   Negative  error
       codes  are  also  returned  by other functions, and are documented with
       them. The codes are given names in the header file. If UTF checking  is
       in force and an invalid UTF subject string is detected, one of a number
       of  UTF-specific negative error codes is returned. Details are given in
       the pcre2unicode page. The following are the other errors that  may  be
       returned by pcre2_match():

         PCRE2_ERROR_NOMATCH

       The subject string did not match the pattern.

         PCRE2_ERROR_PARTIAL

       The  subject  string did not match, but it did match partially. See the
       pcre2partial documentation for details of partial matching.

         PCRE2_ERROR_BADMAGIC

       PCRE2 stores a 4-byte "magic number" at the start of the compiled code,
       to catch the case when it is passed a junk pointer. This is  the  error
       that is returned when the magic number is not present.

         PCRE2_ERROR_BADMODE

       This  error is given when a compiled pattern is passed to a function in
       a library of a different code unit width, for example, a  pattern  com-
       piled  by  the  8-bit  library  is passed to a 16-bit or 32-bit library
       function.

         PCRE2_ERROR_BADOFFSET

       The value of startoffset was greater than the length of the subject.

         PCRE2_ERROR_BADOPTION

       An unrecognized bit was set in the options argument.

         PCRE2_ERROR_BADUTFOFFSET

       The UTF code unit sequence that was passed as a subject was checked and
       found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but  the
       value  of startoffset did not point to the beginning of a UTF character
       or the end of the subject.

         PCRE2_ERROR_CALLOUT

       This error is never generated by pcre2_match() itself. It  is  provided
       for  use  by  callout  functions  that  want  to cause pcre2_match() or
       pcre2_callout_enumerate() to return a distinctive error code.  See  the
       pcre2callout documentation for details.

         PCRE2_ERROR_DEPTHLIMIT

       The nested backtracking depth limit was reached.

         PCRE2_ERROR_HEAPLIMIT

       The heap limit was reached.

         PCRE2_ERROR_INTERNAL

       An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE2 or by overwriting of the compiled pattern.

         PCRE2_ERROR_JIT_STACKLIMIT

       This error is returned when a pattern that was successfully studied us-
       ing JIT is being matched, but the memory available for the just-in-time
       processing stack is not large enough. See  the  pcre2jit  documentation
       for more details.

         PCRE2_ERROR_MATCHLIMIT

       The backtracking match limit was reached.

         PCRE2_ERROR_NOMEMORY

       Heap  memory  is  used  to  remember backtracking points. This error is
       given when the memory allocation function (default  or  custom)  fails.
       Note  that  a  different  error, PCRE2_ERROR_HEAPLIMIT, is given if the
       amount of memory needed exceeds the heap limit. PCRE2_ERROR_NOMEMORY is
       also returned if PCRE2_COPY_MATCHED_SUBJECT is set and  memory  alloca-
       tion fails.

         PCRE2_ERROR_NULL

       Either the code, subject, or match_data argument was passed as NULL.

         PCRE2_ERROR_RECURSELOOP

       This  error  is  returned  when  pcre2_match() detects a recursion loop
       within the pattern. Specifically, it means that either the  whole  pat-
       tern or a capture group has been called recursively for the second time
       at  the  same position in the subject string. Some simple patterns that
       might do this are detected and faulted at compile time, but  more  com-
       plicated  cases,  in particular mutual recursions between two different
       groups, cannot be detected until matching is attempted.


OBTAINING A TEXTUAL ERROR MESSAGE

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       A text message for an error code  from  any  PCRE2  function  (compile,
       match,  or  auxiliary)  can be obtained by calling pcre2_get_error_mes-
       sage(). The code is passed as the first argument,  with  the  remaining
       two  arguments  specifying  a  code  unit buffer and its length in code
       units, into which the text message is placed. The message  is  returned
       in  code  units  of the appropriate width for the library that is being
       used.

       The returned message is terminated with a trailing zero, and the  func-
       tion  returns  the  number  of  code units used, excluding the trailing
       zero. If the error number is unknown, the negative error code PCRE2_ER-
       ROR_BADDATA is returned. If the buffer is too  small,  the  message  is
       truncated (but still with a trailing zero), and the negative error code
       PCRE2_ERROR_NOMEMORY  is returned.  None of the messages are very long;
       a buffer size of 120 code units is ample.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       Captured substrings can be accessed directly by using  the  ovector  as
       described above.  For convenience, auxiliary functions are provided for
       extracting   captured  substrings  as  new,  separate,  zero-terminated
       strings. A substring that contains a binary zero is correctly extracted
       and has a further zero added on the end, but  the  result  is  not,  of
       course, a C string.

       The functions in this section identify substrings by number. The number
       zero refers to the entire matched substring, with higher numbers refer-
       ring  to  substrings  captured by parenthesized groups. After a partial
       match, only substring zero is available.  An  attempt  to  extract  any
       other  substring  gives the error PCRE2_ERROR_PARTIAL. The next section
       describes similar functions for extracting captured substrings by name.

       If a pattern uses the \K escape sequence within a  positive  assertion,
       the reported start of a successful match can be greater than the end of
       the  match.   For  example,  if the pattern (?=ab\K) is matched against
       "ab", the start and end offset values for the match are  2  and  0.  In
       this  situation,  calling  these functions with a zero substring number
       extracts a zero-length empty string.

       You can find the length in code units of a captured  substring  without
       extracting  it  by calling pcre2_substring_length_bynumber(). The first
       argument is a pointer to the match data block, the second is the  group
       number,  and the third is a pointer to a variable into which the length
       is placed. If you just want to know whether or not  the  substring  has
       been captured, you can pass the third argument as NULL.

       The  pcre2_substring_copy_bynumber()  function  copies  a captured sub-
       string into a supplied buffer,  whereas  pcre2_substring_get_bynumber()
       copies  it  into  new memory, obtained using the same memory allocation
       function that was used for the match data block. The  first  two  argu-
       ments  of  these  functions are a pointer to the match data block and a
       capture group number.

       The final arguments of pcre2_substring_copy_bynumber() are a pointer to
       the buffer and a pointer to a variable that contains its length in code
       units.  This is updated to contain the actual number of code units used
       for the extracted substring, excluding the terminating zero.

       For pcre2_substring_get_bynumber() the third and fourth arguments point
       to variables that are updated with a pointer to the new memory and  the
       number  of  code units that comprise the substring, again excluding the
       terminating zero. When the substring is no longer  needed,  the  memory
       should be freed by calling pcre2_substring_free().

       The  return  value  from  all these functions is zero for success, or a
       negative error code. If the pattern match  failed,  the  match  failure
       code  is returned.  If a substring number greater than zero is used af-
       ter a partial match, PCRE2_ERROR_PARTIAL is  returned.  Other  possible
       error codes are:

         PCRE2_ERROR_NOMEMORY

       The  buffer  was  too small for pcre2_substring_copy_bynumber(), or the
       attempt to get memory failed for pcre2_substring_get_bynumber().

         PCRE2_ERROR_NOSUBSTRING

       There is no substring with that number in the  pattern,  that  is,  the
       number is greater than the number of capturing parentheses.

         PCRE2_ERROR_UNAVAILABLE

       The substring number, though not greater than the number of captures in
       the pattern, is greater than the number of slots in the ovector, so the
       substring could not be captured.

         PCRE2_ERROR_UNSET

       The  substring  did  not  participate in the match. For example, if the
       pattern is (abc)|(def) and the subject is "def", and the  ovector  con-
       tains at least two capturing slots, substring number 1 is unset.


EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       The  pcre2_substring_list_get()  function  extracts  all available sub-
       strings and builds a list of pointers to  them.  It  also  (optionally)
       builds  a  second list that contains their lengths (in code units), ex-
       cluding a terminating zero that is added to each of them. All  this  is
       done in a single block of memory that is obtained using the same memory
       allocation function that was used to get the match data block.

       This  function  must be called only after a successful match. If called
       after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.

       The address of the memory block is returned via listptr, which is  also
       the start of the list of string pointers. The end of the list is marked
       by  a  NULL pointer. The address of the list of lengths is returned via
       lengthsptr. If your strings do not contain binary zeros and you do  not
       therefore need the lengths, you may supply NULL as the lengthsptr argu-
       ment  to  disable  the  creation of a list of lengths. The yield of the
       function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the  mem-
       ory  block could not be obtained. When the list is no longer needed, it
       should be freed by calling pcre2_substring_list_free().

       If this function encounters a substring that is unset, which can happen
       when capture group number n+1 matches some part  of  the  subject,  but
       group  n has not been used at all, it returns an empty string. This can
       be distinguished from a genuine zero-length substring by inspecting the
       appropriate offset in the ovector, which contain PCRE2_UNSET for  unset
       substrings, or by calling pcre2_substring_length_bynumber().


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern:

         (a+)b(?<xxx>\d+)...

       the number of the capture group called "xxx" is 2. If the name is known
       to be unique (PCRE2_DUPNAMES was not set), you can find the number from
       the name by calling pcre2_substring_number_from_name(). The first argu-
       ment is the compiled pattern, and the second is the name. The yield  of
       the  function  is the group number, PCRE2_ERROR_NOSUBSTRING if there is
       no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if  there  is
       more  than one group with that name.  Given the number, you can extract
       the substring directly from the ovector, or use one of  the  "bynumber"
       functions described above.

       For  convenience,  there are also "byname" functions that correspond to
       the "bynumber" functions, the only difference being that the second ar-
       gument is a name instead of a number.  If  PCRE2_DUPNAMES  is  set  and
       there are duplicate names, these functions scan all the groups with the
       given  name,  and  return  the  captured substring from the first named
       group that is set.

       If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING  is
       returned.  If  all  groups  with the name have numbers that are greater
       than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is re-
       turned. If there is at least one group with a slot in the ovector,  but
       no group is found to be set, PCRE2_ERROR_UNSET is returned.

       Warning: If the pattern uses the (?| feature to set up multiple capture
       groups  with  the same number, as described in the section on duplicate
       group numbers in the pcre2pattern page, you cannot use names to distin-
       guish the different capture groups, because names are not  included  in
       the  compiled  code.  The  matching process uses only numbers. For this
       reason, the use of different names for  groups  with  the  same  number
       causes an error at compile time.


CREATING A NEW STRING WITH SUBSTITUTIONS

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacement,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);

       This  function  optionally calls pcre2_match() and then makes a copy of
       the subject string in outputbuffer, replacing parts that  were  matched
       with the replacement string, whose length is supplied in rlength, which
       can  be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As
       a special case, if replacement is NULL and rlength  is  zero,  the  re-
       placement  is assumed to be an empty string. If rlength is non-zero, an
       error occurs if replacement is NULL.

       There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to re-
       turn just the replacement string(s). The default action is  to  perform
       just  one  replacement  if  the pattern matches, but there is an option
       that requests multiple replacements  (see  PCRE2_SUBSTITUTE_GLOBAL  be-
       low).

       If  successful,  pcre2_substitute() returns the number of substitutions
       that were carried out. This may be zero if no match was found,  and  is
       never  greater  than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A nega-
       tive value is returned if an error is detected.

       Matches in which a \K item in a lookahead in  the  pattern  causes  the
       match  to  end  before it starts are not supported, and give rise to an
       error return. For global replacements, matches in which \K in a lookbe-
       hind causes the match to start earlier than the point that was  reached
       in the previous iteration are also not supported.

       The  first  seven  arguments  of pcre2_substitute() are the same as for
       pcre2_match(), except that the partial matching options are not permit-
       ted, and match_data may be passed as NULL, in which case a  match  data
       block  is obtained and freed within this function, using memory manage-
       ment functions from the match context, if provided, or else those  that
       were used to allocate memory for the compiled code.

       If  match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set, the
       provided block is used for all calls to pcre2_match(), and its contents
       afterwards are the result of the final call. For global  changes,  this
       will always be a no-match error. The contents of the ovector within the
       match data block may or may not have been changed.

       As  well as the usual options for pcre2_match(), a number of additional
       options can be set in the options argument of pcre2_substitute().   One
       such  option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external
       match_data block must be provided, and it must have already  been  used
       for an external call to pcre2_match() with the same pattern and subject
       arguments.  The  data in the match_data block (return code, offset vec-
       tor) is then  used  for  the  first  substitution  instead  of  calling
       pcre2_match()  from  within pcre2_substitute(). This allows an applica-
       tion to check for a match before choosing to substitute, without having
       to repeat the match.

       The contents of the  externally  supplied  match  data  block  are  not
       changed   when   PCRE2_SUBSTITUTE_MATCHED   is  set.  If  PCRE2_SUBSTI-
       TUTE_GLOBAL is also set, pcre2_match() is called after the  first  sub-
       stitution  to  check for further matches, but this is done using an in-
       ternally obtained match data block, thus always  leaving  the  external
       block unchanged.

       The  code  argument is not used for matching before the first substitu-
       tion when PCRE2_SUBSTITUTE_MATCHED is set, but  it  must  be  provided,
       even  when  PCRE2_SUBSTITUTE_GLOBAL is not set, because it contains in-
       formation such as the UTF setting and the number of capturing parenthe-
       ses in the pattern.

       The default action of pcre2_substitute() is to return  a  copy  of  the
       subject string with matched substrings replaced. However, if PCRE2_SUB-
       STITUTE_REPLACEMENT_ONLY  is  set,  only the replacement substrings are
       returned. In the global case, multiple replacements are concatenated in
       the output buffer. Substitution callouts (see below)  can  be  used  to
       separate them if necessary.

       The  outlengthptr  argument of pcre2_substitute() must point to a vari-
       able that contains the length, in code units, of the output buffer.  If
       the  function is successful, the value is updated to contain the length
       in code units of the new string, excluding the trailing  zero  that  is
       automatically added.

       If  the  function is not successful, the value set via outlengthptr de-
       pends on the type of  error.  For  syntax  errors  in  the  replacement
       string, the value is the offset in the replacement string where the er-
       ror  was  detected.  For  other errors, the value is PCRE2_UNSET by de-
       fault. This includes the case of the output buffer being too small, un-
       less PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set.

       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when  the  output
       buffer is too small. The default action is to return PCRE2_ERROR_NOMEM-
       ORY  immediately.  If  this  option is set, however, pcre2_substitute()
       continues to go through the motions of matching and substituting (with-
       out, of course, writing anything) in  order  to  compute  the  size  of
       buffer  that  is needed. This value is passed back via the outlengthptr
       variable, with  the  result  of  the  function  still  being  PCRE2_ER-
       ROR_NOMEMORY.

       Passing  a  buffer  size  of zero is a permitted way of finding out how
       much memory is needed for given substitution. However, this  does  mean
       that the entire operation is carried out twice. Depending on the appli-
       cation,  it  may  be more efficient to allocate a large buffer and free
       the  excess  afterwards,  instead   of   using   PCRE2_SUBSTITUTE_OVER-
       FLOW_LENGTH.

       The  replacement  string,  which  is interpreted as a UTF string in UTF
       mode, is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set.  An
       invalid UTF replacement string causes an immediate return with the rel-
       evant UTF error code.

       If  PCRE2_SUBSTITUTE_LITERAL  is set, the replacement string is not in-
       terpreted in any way. By default, however, a dollar character is an es-
       cape character that can specify the insertion of characters  from  cap-
       ture  groups  and names from (*MARK) or other control verbs in the pat-
       tern. Dollar is the only escape character (backslash is treated as lit-
       eral). The following forms are always recognized:

         $$                  insert a dollar character
         $<n> or ${<n>}      insert the contents of group <n>
         $*MARK or ${*MARK}  insert a control verb name

       Either a group number or a group name  can  be  given  for  <n>.  Curly
       brackets  are  required only if the following character would be inter-
       preted as part of the number or name. The number may be zero to include
       the entire matched string.   For  example,  if  the  pattern  a(b)c  is
       matched  with "=abc=" and the replacement string "+$1$0$1+", the result
       is "=+babcb+=".

       $*MARK inserts the name from the last encountered backtracking  control
       verb  on the matching path that has a name. (*MARK) must always include
       a name, but the other verbs need not.  For  example,  in  the  case  of
       (*MARK:A)(*PRUNE) the name inserted is "A", but for (*MARK:A)(*PRUNE:B)
       the  relevant  name is "B". This facility can be used to perform simple
       simultaneous substitutions, as this pcre2test example shows:

         /(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK}
             apple lemon
          2: pear orange

       PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
       string, replacing every matching substring. If this option is not  set,
       only  the  first matching substring is replaced. The search for matches
       takes place in the original subject string (that is, previous  replace-
       ments  do  not  affect  it).  Iteration is implemented by advancing the
       startoffset value for each search, which is always  passed  the  entire
       subject string. If an offset limit is set in the match context, search-
       ing stops when that limit is reached.

       You  can  restrict  the effect of a global substitution to a portion of
       the subject string by setting either or both of startoffset and an off-
       set limit. Here is a pcre2test example:

         /B/g,replace=!,use_offset_limit
         ABC ABC ABC ABC\=offset=3,offset_limit=12
          2: ABC A!C A!C ABC

       When continuing with global substitutions after  matching  a  substring
       with zero length, an attempt to find a non-empty match at the same off-
       set is performed.  If this is not successful, the offset is advanced by
       one character except when CRLF is a valid newline sequence and the next
       two  characters are CR, LF. In this case, the offset is advanced by two
       characters.

       PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that
       do not appear in the pattern to be treated as unset groups. This option
       should be used with care, because it means that a typo in a group  name
       or number no longer causes the PCRE2_ERROR_NOSUBSTRING error.

       PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including un-
       known  groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated
       as empty strings when inserted as described above. If  this  option  is
       not set, an attempt to insert an unset group causes the PCRE2_ERROR_UN-
       SET  error.  This  option  does not influence the extended substitution
       syntax described below.

       PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to  the
       replacement  string.  Without this option, only the dollar character is
       special, and only the group insertion forms  listed  above  are  valid.
       When PCRE2_SUBSTITUTE_EXTENDED is set, two things change:

       Firstly,  backslash in a replacement string is interpreted as an escape
       character. The usual forms such as \n or \x{ddd} can be used to specify
       particular character codes, and backslash followed by any  non-alphanu-
       meric  character  quotes  that character. Extended quoting can be coded
       using \Q...\E, exactly as in pattern strings.

       There are also four escape sequences for forcing the case  of  inserted
       letters.   The  insertion  mechanism has three states: no case forcing,
       force upper case, and force lower case. The escape sequences change the
       current state: \U and \L change to upper or lower case forcing, respec-
       tively, and \E (when not terminating a \Q quoted sequence)  reverts  to
       no  case  forcing. The sequences \u and \l force the next character (if
       it is a letter) to upper or lower  case,  respectively,  and  then  the
       state automatically reverts to no case forcing. Case forcing applies to
       all  inserted  characters, including those from capture groups and let-
       ters within \Q...\E quoted sequences. If either PCRE2_UTF or  PCRE2_UCP
       was  set when the pattern was compiled, Unicode properties are used for
       case forcing characters whose code points are greater than 127.

       Note that case forcing sequences such as \U...\E do not nest. For exam-
       ple, the result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc";  the  final
       \E  has  no  effect.  Note  also  that the PCRE2_ALT_BSUX and PCRE2_EX-
       TRA_ALT_BSUX options do not apply to replacement strings.

       The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to  add  more
       flexibility  to  capture  group  substitution. The syntax is similar to
       that used by Bash:

         ${<n>:-<string>}
         ${<n>:+<string1>:<string2>}

       As before, <n> may be a group number or a name. The first  form  speci-
       fies  a  default  value. If group <n> is set, its value is inserted; if
       not, <string> is expanded and the  result  inserted.  The  second  form
       specifies  strings that are expanded and inserted when group <n> is set
       or unset, respectively. The first form is just a  convenient  shorthand
       for

         ${<n>:+${<n>}:<string>}

       Backslash  can  be  used to escape colons and closing curly brackets in
       the replacement strings. A change of the case forcing  state  within  a
       replacement  string  remains  in  force  afterwards,  as  shown in this
       pcre2test example:

         /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo
             body
          1: hello
             somebody
          1: HELLO

       The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these  extended
       substitutions.  However,  PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause un-
       known groups in the extended syntax forms to be treated as unset.

       If  PCRE2_SUBSTITUTE_LITERAL  is  set,  PCRE2_SUBSTITUTE_UNKNOWN_UNSET,
       PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are irrele-
       vant and are ignored.

   Substitution errors

       In  the  event of an error, pcre2_substitute() returns a negative error
       code. Except for PCRE2_ERROR_NOMATCH (which is never returned),  errors
       from pcre2_match() are passed straight back.

       PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser-
       tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.

       PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ-
       ing  an  unknown  substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set)
       when the simple (non-extended) syntax is used and  PCRE2_SUBSTITUTE_UN-
       SET_EMPTY is not set.

       PCRE2_ERROR_NOMEMORY  is  returned  if  the  output  buffer  is not big
       enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size
       of buffer that is needed is returned via outlengthptr. Note  that  this
       does not happen by default.

       PCRE2_ERROR_NULL is returned if PCRE2_SUBSTITUTE_MATCHED is set but the
       match_data  argument is NULL or if the subject or replacement arguments
       are NULL. For backward compatibility reasons an exception is  made  for
       the replacement argument if the rlength argument is also 0.

       PCRE2_ERROR_BADREPLACEMENT  is  used for miscellaneous syntax errors in
       the replacement string, with more  particular  errors  being  PCRE2_ER-
       ROR_BADREPESCAPE (invalid escape sequence), PCRE2_ERROR_REPMISSINGBRACE
       (closing  curly bracket not found), PCRE2_ERROR_BADSUBSTITUTION (syntax
       error in extended group substitution),  and  PCRE2_ERROR_BADSUBSPATTERN
       (the pattern match ended before it started or the match started earlier
       than  the  current  position  in the subject, which can happen if \K is
       used in an assertion).

       As for all PCRE2 errors, a text message that describes the error can be
       obtained by calling the pcre2_get_error_message()  function  (see  "Ob-
       taining a textual error message" above).

   Substitution callouts

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       The  pcre2_set_substitution_callout() function can be used to specify a
       callout function for pcre2_substitute(). This information is passed  in
       a match context. The callout function is called after each substitution
       has been processed, but it can cause the replacement not to happen. The
       callout  function is not called for simulated substitutions that happen
       as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option.

       The first argument of the callout function is a pointer to a substitute
       callout block structure, which contains the following fields, not  nec-
       essarily in this order:

         uint32_t    version;
         uint32_t    subscount;
         PCRE2_SPTR  input;
         PCRE2_SPTR  output;
         PCRE2_SIZE *ovector;
         uint32_t    oveccount;
         PCRE2_SIZE  output_offsets[2];

       The  version field contains the version number of the block format. The
       current version is 0. The version number will  increase  in  future  if
       more  fields are added, but the intention is never to remove any of the
       existing fields.

       The subscount field is the number of the current match. It is 1 for the
       first callout, 2 for the second, and so on. The input and output point-
       ers are copies of the values passed to pcre2_substitute().

       The ovector field points to the ovector, which contains the  result  of
       the most recent match. The oveccount field contains the number of pairs
       that are set in the ovector, and is always greater than zero.

       The  output_offsets  vector  contains the offsets of the replacement in
       the output string. This has already been processed for dollar  and  (if
       requested) backslash substitutions as described above.

       The  second  argument  of  the  callout function is the value passed as
       callout_data when the function was registered. The  value  returned  by
       the callout function is interpreted as follows:

       If  the  value is zero, the replacement is accepted, and, if PCRE2_SUB-
       STITUTE_GLOBAL is set, processing continues with a search for the  next
       match.  If  the  value  is not zero, the current replacement is not ac-
       cepted. If the value is greater than zero,  processing  continues  when
       PCRE2_SUBSTITUTE_GLOBAL  is set. Otherwise (the value is less than zero
       or PCRE2_SUBSTITUTE_GLOBAL is not set), the rest of the input is copied
       to the output and the call to pcre2_substitute() exits,  returning  the
       number of matches so far.


DUPLICATE CAPTURE GROUP NAMES

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       When  a  pattern  is compiled with the PCRE2_DUPNAMES option, names for
       capture groups are not required to be unique. Duplicate names  are  al-
       ways  allowed for groups with the same number, created by using the (?|
       feature. Indeed, if such groups are named, they are required to use the
       same names.

       Normally, patterns that use duplicate names are such that  in  any  one
       match,  only  one of each set of identically-named groups participates.
       An example is shown in the pcre2pattern documentation.

       When  duplicates   are   present,   pcre2_substring_copy_byname()   and
       pcre2_substring_get_byname()  return  the first substring corresponding
       to the given name that is set. Only if none are set is  PCRE2_ERROR_UN-
       SET  is  returned.  The pcre2_substring_number_from_name() function re-
       turns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are  duplicate
       names.

       If  you want to get full details of all captured substrings for a given
       name, you must use the pcre2_substring_nametable_scan()  function.  The
       first  argument is the compiled pattern, and the second is the name. If
       the third and fourth arguments are NULL, the function returns  a  group
       number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

       When the third and fourth arguments are not NULL, they must be pointers
       to  variables  that are updated by the function. After it has run, they
       point to the first and last entries in the name-to-number table for the
       given name, and the function returns the length of each entry  in  code
       units.  In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are
       no entries for the given name.

       The format of the name table is described above in the section entitled
       Information about a pattern. Given all the  relevant  entries  for  the
       name,  you  can  extract  each of their numbers, and hence the captured
       data.


FINDING ALL POSSIBLE MATCHES AT ONE POSITION

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which  stops when it finds the first match at a given point in the sub-
       ject. If you want to find all possible matches, or the longest possible
       match at a given position,  consider  using  the  alternative  matching
       function  (see  below) instead. If you cannot use the alternative func-
       tion, you can kludge it up by making use of the callout facility, which
       is described in the pcre2callout documentation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.  When your callout function is called, extract and save the  cur-
       rent  matched  substring.  Then return 1, which forces pcre2_match() to
       backtrack and try other alternatives. Ultimately, when it runs  out  of
       matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       The  function  pcre2_dfa_match()  is  called  to match a subject string
       against a compiled pattern, using a matching algorithm that  scans  the
       subject string just once (not counting lookaround assertions), and does
       not  backtrack (except when processing lookaround assertions). This has
       different characteristics to the normal algorithm, and is not  compati-
       ble  with  Perl.  Some  of  the features of PCRE2 patterns are not sup-
       ported. Nevertheless, there are times when this kind of matching can be
       useful. For a discussion of the two matching algorithms, and a list  of
       features that pcre2_dfa_match() does not support, see the pcre2matching
       documentation.

       The  arguments  for  the pcre2_dfa_match() function are the same as for
       pcre2_match(), plus two extras. The ovector within the match data block
       is used in a different way, and this is described below. The other com-
       mon arguments are used in the same way as for pcre2_match(),  so  their
       description is not repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple paths through the pattern tree. More work-
       space is needed for patterns and subjects where there are a lot of  po-
       tential matches.

       Here is an example of a simple call to pcre2_dfa_match():

         int wspace[20];
         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_dfa_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL,           /* a match context; NULL means use defaults */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre2_dfa_match()

       The  unused  bits of the options argument for pcre2_dfa_match() must be
       zero.  The  only   bits   that   may   be   set   are   PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT,  PCRE2_ENDANCHORED, PCRE2_NOTBOL, PCRE2_NO-
       TEOL,   PCRE2_NOTEMPTY,   PCRE2_NOTEMPTY_ATSTART,   PCRE2_NO_UTF_CHECK,
       PCRE2_PARTIAL_HARD,    PCRE2_PARTIAL_SOFT,    PCRE2_DFA_SHORTEST,   and
       PCRE2_DFA_RESTART. All but the last four of these are exactly the  same
       as for pcre2_match(), so their description is not repeated here.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These  have  the  same general effect as they do for pcre2_match(), but
       the details are slightly different. When PCRE2_PARTIAL_HARD is set  for
       pcre2_dfa_match(),  it  returns  PCRE2_ERROR_PARTIAL  if the end of the
       subject is reached and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches have already been found. When PCRE2_PARTIAL_SOFT  is  set,  the
       return  code  PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL
       if the end of the subject is  reached,  there  have  been  no  complete
       matches, but there is still at least one matching possibility. The por-
       tion  of  the  string that was inspected when the longest partial match
       was found is set as the first matching string in both cases. There is a
       more detailed discussion of partial and  multi-segment  matching,  with
       examples, in the pcre2partial documentation.

         PCRE2_DFA_SHORTEST

       Setting  the PCRE2_DFA_SHORTEST option causes the matching algorithm to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE2_DFA_RESTART

       When  pcre2_dfa_match() returns a partial match, it is possible to call
       it again, with additional subject characters, and have it continue with
       the same match. The PCRE2_DFA_RESTART option requests this action; when
       it is set, the workspace and wscount options must  reference  the  same
       vector  as  before  because data about the match so far is left in them
       after a partial match. There is more discussion of this facility in the
       pcre2partial documentation.

   Successful returns from pcre2_dfa_match()

       When pcre2_dfa_match() succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something> <something else> <something further>
         <something> <something else>
         <something>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number  of  matched substrings. The offsets of the sub-
       strings are returned in the ovector, and can be extracted by number  in
       the  same way as for pcre2_match(), but the numbers bear no relation to
       any capture groups that may exist in the pattern, because DFA  matching
       does not support capturing.

       Calls  to the convenience functions that extract substrings by name re-
       turn the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used af-
       ter a DFA match. The convenience functions that extract  substrings  by
       number never return PCRE2_ERROR_NOSUBSTRING.

       The  matched  strings  are  stored  in  the ovector in reverse order of
       length; that is, the longest matching string is first.  If  there  were
       too  many matches to fit into the ovector, the yield of the function is
       zero, and the vector is filled with the longest matches.

       NOTE: PCRE2's "auto-possessification" optimization usually  applies  to
       character  repeats at the end of a pattern (as well as internally). For
       example, the pattern "a\d+" is compiled as if it were "a\d++". For  DFA
       matching,  this means that only one possible match is found. If you re-
       ally do want multiple matches in such cases, either use an ungreedy re-
       peat such as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when  com-
       piling.

   Error returns from pcre2_dfa_match()

       The pcre2_dfa_match() function returns a negative number when it fails.
       Many  of  the  errors  are  the same as for pcre2_match(), as described
       above.  There are in addition the following errors that are specific to
       pcre2_dfa_match():

         PCRE2_ERROR_DFA_UITEM

       This return is given if pcre2_dfa_match() encounters  an  item  in  the
       pattern  that it does not support, for instance, the use of \C in a UTF
       mode or a backreference.

         PCRE2_ERROR_DFA_UCOND

       This return is given if pcre2_dfa_match() encounters a  condition  item
       that uses a backreference for the condition, or a test for recursion in
       a specific capture group. These are not supported.

         PCRE2_ERROR_DFA_UINVALID_UTF

       This  return is given if pcre2_dfa_match() is called for a pattern that
       was compiled with PCRE2_MATCH_INVALID_UTF. This is  not  supported  for
       DFA matching.

         PCRE2_ERROR_DFA_WSSIZE

       This  return  is  given  if  pcre2_dfa_match() runs out of space in the
       workspace vector.

         PCRE2_ERROR_DFA_RECURSE

       When a recursion or subroutine call is processed, the matching function
       calls itself recursively, using private  memory  for  the  ovector  and
       workspace.   This  error  is given if the internal ovector is not large
       enough. This should be extremely rare, as a  vector  of  size  1000  is
       used.

         PCRE2_ERROR_DFA_BADRESTART

       When  pcre2_dfa_match()  is  called  with the PCRE2_DFA_RESTART option,
       some plausibility checks are made on the  contents  of  the  workspace,
       which  should  contain data about the previous partial match. If any of
       these checks fail, this error is given.


SEE ALSO

       pcre2build(3),   pcre2callout(3),    pcre2demo(3),    pcre2matching(3),
       pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 24 April 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.44                      24 April 2024                     PCRE2API(3)
------------------------------------------------------------------------------



PCRE2BUILD(3)              Library Functions Manual              PCRE2BUILD(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


BUILDING PCRE2

       PCRE2  is distributed with a configure script that can be used to build
       the library in Unix-like environments using the applications  known  as
       Autotools. Also in the distribution are files to support building using
       CMake  instead  of configure. The text file README contains general in-
       formation about building with Autotools (some of which is repeated  be-
       low),  and  also  has some comments about building on various operating
       systems. The files in the vms directory support building under OpenVMS.
       There is a lot more information about building PCRE2 without using  Au-
       totools  (including  information  about  using  CMake  and building "by
       hand") in the text file called NON-AUTOTOOLS-BUILD.  You should consult
       this file as well as the README file if you are building in a non-Unix-
       like environment.


PCRE2 BUILD-TIME OPTIONS

       The rest of this document describes the optional features of PCRE2 that
       can be selected when the library is compiled. It  assumes  use  of  the
       configure  script,  where  the  optional features are selected or dese-
       lected by providing options to configure before running the  make  com-
       mand.  However,  the same options can be selected in both Unix-like and
       non-Unix-like environments if you are using CMake instead of  configure
       to build PCRE2.

       If  you  are not using Autotools or CMake, option selection can be done
       by editing the config.h file, or by passing parameter settings  to  the
       compiler, as described in NON-AUTOTOOLS-BUILD.

       The complete list of options for configure (which includes the standard
       ones  such  as  the selection of the installation directory) can be ob-
       tained by running

         ./configure --help

       The following sections include descriptions of "on/off"  options  whose
       names begin with --enable or --disable. Because of the way that config-
       ure  works, --enable and --disable always come in pairs, so the comple-
       mentary option always exists as well, but as it specifies the  default,
       it is not described.  Options that specify values have names that start
       with --with. At the end of a configure run, a summary of the configura-
       tion is output.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES

       By  default, a library called libpcre2-8 is built, containing functions
       that take string arguments contained in arrays  of  bytes,  interpreted
       either  as single-byte characters, or UTF-8 strings. You can also build
       two other libraries, called libpcre2-16 and libpcre2-32, which  process
       strings  that  are contained in arrays of 16-bit and 32-bit code units,
       respectively. These can be interpreted either as single-unit characters
       or UTF-16/UTF-32 strings. To build these additional libraries, add  one
       or both of the following to the configure command:

         --enable-pcre2-16
         --enable-pcre2-32

       If you do not want the 8-bit library, add

         --disable-pcre2-8

       as  well.  At least one of the three libraries must be built. Note that
       the POSIX wrapper is for the 8-bit library only, and that pcre2grep  is
       an  8-bit  program.  Neither  of these are built if you select only the
       16-bit or 32-bit libraries.


BUILDING SHARED AND STATIC LIBRARIES

       The Autotools PCRE2 building process uses libtool to build both  shared
       and  static  libraries by default. You can suppress an unwanted library
       by adding one of

         --disable-shared
         --disable-static

       to the configure command. Setting --disable-shared ensures  that  PCRE2
       libraries  are  built  as  static libraries. The binaries that are then
       created as part of  the  build  process  (for  example,  pcre2test  and
       pcre2grep)  are linked statically with one or more PCRE2 libraries, but
       may also be dynamically linked with other libraries such  as  libc.  If
       you  want these binaries to be fully statically linked, you can set LD-
       FLAGS like this:

       LDFLAGS=--static ./configure --disable-shared

       Note the two hyphens in --static. Of course, this works only if  static
       versions of all the relevant libraries are available for linking.


UNICODE AND UTF SUPPORT

       By  default,  PCRE2 is built with support for Unicode and UTF character
       strings.  To build it without Unicode support, add

         --disable-unicode

       to the configure command. This setting applies to all three  libraries.
       It  is  not  possible to build one library with Unicode support and an-
       other without in the same configuration.

       Of itself, Unicode support does not make PCRE2 treat strings as  UTF-8,
       UTF-16 or UTF-32. To do that, applications that use the library can set
       the  PCRE2_UTF  option when they call pcre2_compile() to compile a pat-
       tern.  Alternatively, patterns may be started with  (*UTF)  unless  the
       application has locked this out by setting PCRE2_NEVER_UTF.

       UTF support allows the libraries to process character code points up to
       0x10ffff  in  the  strings that they handle. Unicode support also gives
       access to the Unicode properties of characters, using  pattern  escapes
       such as \P, \p, and \X. Only the general category properties such as Lu
       and Nd, script names, and some bi-directional properties are supported.
       Details are given in the pcre2pattern documentation.

       Pattern escapes such as \d and \w do not by default make use of Unicode
       properties.  The  application  can  request that they do by setting the
       PCRE2_UCP option. Unless the application  has  set  PCRE2_NEVER_UCP,  a
       pattern may also request this by starting with (*UCP).


DISABLING THE USE OF \C

       The \C escape sequence, which matches a single code unit, even in a UTF
       mode,  can  cause unpredictable behaviour because it may leave the cur-
       rent matching point in the middle of a multi-code-unit  character.  The
       application  can lock it out by setting the PCRE2_NEVER_BACKSLASH_C op-
       tion when calling pcre2_compile(). There is also a build-time option

         --enable-never-backslash-C

       (note the upper case C) which locks out the use of \C entirely.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time (JIT) compiler support is included in the build by  speci-
       fying

         --enable-jit

       This  support  is available only for certain hardware architectures. If
       this option is set for an unsupported architecture,  a  building  error
       occurs.  If in doubt, use

         --enable-jit=auto

       which  enables  JIT  only if the current hardware is supported. You can
       check if JIT is enabled in the configuration summary that is output  at
       the  end  of a configure run. If you are enabling JIT under SELinux you
       may also want to add

         --enable-jit-sealloc

       which enables the use of an execmem allocator in JIT that is compatible
       with SELinux. This has no  effect  if  JIT  is  not  enabled.  See  the
       pcre2jit  documentation for a discussion of JIT usage. When JIT support
       is enabled, pcre2grep automatically makes use of it, unless you add

         --disable-pcre2grep-jit

       to the configure command.


NEWLINE RECOGNITION

       By default, PCRE2 interprets the linefeed (LF) character as  indicating
       the  end  of  a line. This is the normal newline character on Unix-like
       systems. You can compile PCRE2 to use carriage return (CR) instead,  by
       adding

         --enable-newline-is-cr

       to  the  configure command. There is also an --enable-newline-is-lf op-
       tion, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two-character sequence CRLF (CR immediately followed by LF). If you
       want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which causes PCRE2 to recognize any of the three sequences CR,  LF,  or
       CRLF as indicating a line ending. A fifth option, specified by

         --enable-newline-is-any

       causes  PCRE2  to  recognize  any Unicode newline sequence. The Unicode
       newline sequences are the three just mentioned, plus the single charac-
       ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
       U+0085), LS (line separator,  U+2028),  and  PS  (paragraph  separator,
       U+2029). The final option is

         --enable-newline-is-nul

       which  causes  NUL  (binary  zero) to be set as the default line-ending
       character.

       Whatever default line ending convention is selected when PCRE2 is built
       can be overridden by applications that use the library. At  build  time
       it is recommended to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, independently of what has been selected as  the  line  ending
       sequence. If you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE2 is built can be overridden by  applications
       that use the library.


HANDLING VERY LARGE PATTERNS

       Within  a  compiled  pattern,  offset values are used to point from one
       part to another (for example, from an opening parenthesis to an  alter-
       nation  metacharacter).  By default, in the 8-bit and 16-bit libraries,
       two-byte values are used for these offsets, leading to a  maximum  size
       for a compiled pattern of around 64 thousand code units. This is suffi-
       cient  to handle all but the most gigantic patterns. Nevertheless, some
       people do want to process truly enormous patterns, so it is possible to
       compile PCRE2 to use three-byte or four-byte offsets by adding  a  set-
       ting such as

         --with-link-size=3

       to  the  configure command. The value given must be 2, 3, or 4. For the
       16-bit library, a value of 3 is rounded up to 4.  In  these  libraries,
       using  longer  offsets slows down the operation of PCRE2 because it has
       to load additional data when handling them. For the 32-bit library  the
       value  is  always 4 and cannot be overridden; the value of --with-link-
       size is ignored.


LIMITING PCRE2 RESOURCE USAGE

       The pcre2_match() function increments a counter each time it goes round
       its main loop. Putting a limit on this counter controls the  amount  of
       computing  resource  used  by a single call to pcre2_match(). The limit
       can be changed at run time, as described in the pcre2api documentation.
       The default is 10 million, but this can be changed by adding a  setting
       such as

         --with-match-limit=500000

       to   the   configure   command.   This  setting  also  applies  to  the
       pcre2_dfa_match() matching function, and to JIT  matching  (though  the
       counting is done differently).

       The  pcre2_match()  function  uses  heap  memory to record backtracking
       points. The more nested backtracking points there  are  (that  is,  the
       deeper  the  search tree), the more memory is needed. There is an upper
       limit, specified in kibibytes (units of 1024 bytes). This limit can  be
       changed  at  run  time, as described in the pcre2api documentation. The
       default limit (in effect unlimited) is 20 million. You can change  this
       by a setting such as

         --with-heap-limit=500

       which  limits the amount of heap to 500 KiB. This limit applies only to
       interpretive matching in pcre2_match() and pcre2_dfa_match(), which may
       also use the heap for internal workspace  when  processing  complicated
       patterns.  This limit does not apply when JIT (which has its own memory
       arrangements) is used.

       You can also explicitly limit the depth of nested backtracking  in  the
       pcre2_match() interpreter. This limit defaults to the value that is set
       for  --with-match-limit.  You  can set a lower default limit by adding,
       for example,

         --with-match-limit-depth=10000

       to the configure command. This value can be  overridden  at  run  time.
       This  depth  limit  indirectly limits the amount of heap memory that is
       used, but because the size of each backtracking "frame" depends on  the
       number  of  capturing parentheses in a pattern, the amount of heap that
       is used before the limit is reached varies  from  pattern  to  pattern.
       This limit was more useful in versions before 10.30, where function re-
       cursion was used for backtracking.

       As well as applying to pcre2_match(), the depth limit also controls the
       depth  of recursive function calls in pcre2_dfa_match(). These are used
       for lookaround assertions, atomic groups,  and  recursion  within  pat-
       terns.  The limit does not apply to JIT matching.


LIMITING VARIABLE-LENGTH LOOKBEHIND ASSERTIONS

       Lookbehind  assertions  in which one or more branches can match a vari-
       able number of characters are supported only  if  there  is  a  maximum
       matching  length  for  each  top-level branch. There is a limit to this
       maximum that defaults to 255 characters. You can alter this default  by
       a setting such as

         --with-max-varlookbehind=100

       The limit can be changed at runtime by calling pcre2_set_max_varlookbe-
       hind().  Lookbehind  assertions  in  which every branch matches a fixed
       number of characters (not necessarily all the same) are not constrained
       by this limit.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE2 uses fixed tables for processing characters whose code points are
       less than 256. By default, PCRE2 is built with a set of tables that are
       distributed in the file src/pcre2_chartables.c.dist. These  tables  are
       for ASCII codes only. If you add

         --enable-rebuild-chartables

       to  the  configure  command, the distributed tables are no longer used.
       Instead, a program called pcre2_dftables is compiled and run. This out-
       puts the source for new set of tables, created in the default locale of
       your C run-time system. This method of replacing the  tables  does  not
       work if you are cross compiling, because pcre2_dftables needs to be run
       on the local host and therefore not compiled with the cross compiler.

       If you need to create alternative tables when cross compiling, you will
       have  to  do so "by hand". There may also be other reasons for creating
       tables manually.  To cause pcre2_dftables to  be  built  on  the  local
       host, run a normal compiling command, and then run the program with the
       output file as its argument, for example:

         cc src/pcre2_dftables.c -o pcre2_dftables
         ./pcre2_dftables src/pcre2_chartables.c

       This  builds the tables in the default locale of the local host. If you
       want to specify a locale, you must use the -L option:

         LC_ALL=fr_FR ./pcre2_dftables -L src/pcre2_chartables.c

       You can also specify -b (with or without -L). This causes the tables to
       be written in binary instead of as source code. A set of binary  tables
       can  be  loaded  into memory by an application and passed to pcre2_com-
       pile() in the same way as tables created by calling pcre2_maketables().
       The tables are just a string of bytes, independent of hardware  charac-
       teristics  such  as  endianness. This means they can be bundled with an
       application that runs in different environments, to  ensure  consistent
       behaviour.


USING EBCDIC CODE

       PCRE2  assumes  by default that it will run in an environment where the
       character code is ASCII or Unicode, which is a superset of ASCII.  This
       is the case for most computer operating systems. PCRE2 can, however, be
       compiled to run in an 8-bit EBCDIC environment by adding

         --enable-ebcdic --disable-unicode

       to the configure command. This setting implies --enable-rebuild-charta-
       bles.  You should only use it if you know that you are in an EBCDIC en-
       vironment (for example, an IBM mainframe operating system).

       It is not possible to support both EBCDIC and UTF-8 codes in  the  same
       version  of  the  library. Consequently, --enable-unicode and --enable-
       ebcdic are mutually exclusive.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the value 0x15 by default. However, in some EBCDIC  environments,  0x25
       is used. In such an environment you should use

         --enable-ebcdic-nl25

       as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR
       has  the  same  value  as in ASCII, namely, 0x0d. Whichever of 0x15 and
       0x25 is not chosen as LF is made to correspond to the Unicode NEL char-
       acter (which, in Unicode, is 0x85).

       The options that select newline behaviour, such as --enable-newline-is-
       cr, and equivalent run-time options, refer to these character values in
       an EBCDIC environment.


PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS

       By default pcre2grep supports the use of callouts with string arguments
       within the patterns it is matching. There are two kinds: one that  gen-
       erates output using local code, and another that calls an external pro-
       gram  or  script.   If --disable-pcre2grep-callout-fork is added to the
       configure command, only the first kind  of  callout  is  supported;  if
       --disable-pcre2grep-callout  is  used,  all callouts are completely ig-
       nored. For more details of pcre2grep callouts, see the pcre2grep  docu-
       mentation.


PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By  default,  pcre2grep reads all files as plain text. You can build it
       so that it recognizes files whose names end in .gz or .bz2,  and  reads
       them with libz or libbz2, respectively, by adding one or both of

         --enable-pcre2grep-libz
         --enable-pcre2grep-libbz2

       to the configure command. These options naturally require that the rel-
       evant  libraries  are installed on your system. Configuration will fail
       if they are not.


PCRE2GREP BUFFER SIZE

       pcre2grep uses an internal buffer to hold a "window" on the file it  is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The default starting size of the buffer is 20KiB. The
       buffer  itself  is  three times this size, but because of the way it is
       used for holding "before" lines, the longest line that is guaranteed to
       be processable is the notional buffer size. If a longer line is encoun-
       tered, pcre2grep automatically expands the buffer, up  to  a  specified
       maximum  size, whose default is 1MiB or the starting size, whichever is
       the larger. You can change the default parameter values by adding,  for
       example,

         --with-pcre2grep-bufsize=51200
         --with-pcre2grep-max-bufsize=2097152

       to  the  configure  command. The caller of pcre2grep can override these
       values by using --buffer-size  and  --max-buffer-size  on  the  command
       line.


PCRE2TEST OPTION FOR LIBREADLINE SUPPORT

       If you add one of

         --enable-pcre2test-libreadline
         --enable-pcre2test-libedit

       to  the configure command, pcre2test is linked with the libreadline or-
       libedit library, respectively, and when its input is from  a  terminal,
       it  reads  it using the readline() function. This provides line-editing
       and history facilities. Note that libreadline is  GPL-licensed,  so  if
       you  distribute  a binary of pcre2test linked in this way, there may be
       licensing issues. These can be avoided by linking instead with libedit,
       which has a BSD licence.

       Setting --enable-pcre2test-libreadline causes the -lreadline option  to
       be  added to the pcre2test build. In many operating environments with a
       system-installed readline library this is sufficient. However, in  some
       environments (e.g. if an unmodified distribution version of readline is
       in  use),  some  extra configuration may be necessary. The INSTALL file
       for libreadline says this:

         "Readline uses the termcap functions, but does not link with
         the termcap or curses library itself, allowing applications
         which link with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


INCLUDING DEBUGGING CODE

       If you add

         --enable-debug

       to  the configure command, additional debugging code is included in the
       build. This feature is intended for use by the PCRE2 maintainers.


DEBUGGING WITH VALGRIND SUPPORT

       If you add

         --enable-valgrind

       to the configure command, PCRE2 will use valgrind annotations  to  mark
       certain  memory  regions as unaddressable. This allows it to detect in-
       valid memory accesses, and is mostly useful for debugging PCRE2 itself.


CODE COVERAGE REPORTING

       If your C compiler is gcc, you can build a version of  PCRE2  that  can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

         --enable-coverage

       to the configure command and build PCRE2 in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage  reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

         CCACHE_DISABLE=1

       before running make to build PCRE2, so that ccache is not used.

       When --enable-coverage is used,  the  following  addition  targets  are
       added to the Makefile:

         make coverage

       This  creates  a  fresh coverage report for the PCRE2 test suite. It is
       equivalent to running "make coverage-reset", "make  coverage-baseline",
       "make check", and then "make coverage-report".

         make coverage-reset

       This zeroes the coverage counters, but does nothing else.

         make coverage-baseline

       This captures baseline coverage information.

         make coverage-report

       This creates the coverage report.

         make coverage-clean-report

       This  removes the generated coverage report without cleaning the cover-
       age data itself.

         make coverage-clean-data

       This removes the captured coverage data without removing  the  coverage
       files created at compile time (*.gcno).

         make coverage-clean

       This  cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and  lcov  docu-
       mentation.


DISABLING THE Z AND T FORMATTING MODIFIERS

       The  C99  standard  defines formatting modifiers z and t for size_t and
       ptrdiff_t values, respectively. By default, PCRE2 uses these  modifiers
       in environments other than old versions of Microsoft Visual Studio when
       __STDC_VERSION__  is  defined  and has a value greater than or equal to
       199901L (indicating support for C99).  However, there is at  least  one
       environment that claims to be C99 but does not support these modifiers.
       If

         --disable-percent-zt

       is specified, no use is made of the z or t modifiers. Instead of %td or
       %zu,  a  suitable  format is used depending in the size of long for the
       platform.


SUPPORT FOR FUZZERS

       There is a special option for use by people who  want  to  run  fuzzing
       tests on PCRE2:

         --enable-fuzz-support

       At present this applies only to the 8-bit library. If set, it causes an
       extra  library  called  libpcre2-fuzzsupport.a to be built, but not in-
       stalled. This contains a single  function  called  LLVMFuzzerTestOneIn-
       put()  whose  arguments are a pointer to a string and the length of the
       string. When called, this function tries to compile  the  string  as  a
       pattern,  and if that succeeds, to match it.  This is done both with no
       options and with some random options bits that are generated  from  the
       string.

       Setting  --enable-fuzz-support  also  causes  a binary called pcre2fuz-
       zcheck to be created. This is normally run under valgrind or used  when
       PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing
       function  and  outputs  information  about  what it is doing. The input
       strings are specified by arguments: if an argument starts with "="  the
       rest  of it is a literal input string. Otherwise, it is assumed to be a
       file name, and the contents of the file are the test string.


OBSOLETE OPTION

       In versions of PCRE2 prior to 10.30, there were two  ways  of  handling
       backtracking  in the pcre2_match() function. The default was to use the
       system stack, but if

         --disable-stack-for-recursion

       was set, memory on the heap was used. From release 10.30  onwards  this
       has  changed  (the  stack  is  no longer used) and this option now does
       nothing except give a warning.


SEE ALSO

       pcre2api(3), pcre2-config(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 15 April 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.44                      15 April 2024                   PCRE2BUILD(3)
------------------------------------------------------------------------------



PCRE2CALLOUT(3)            Library Functions Manual            PCRE2CALLOUT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SYNOPSIS

       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *, void *);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);


DESCRIPTION

       PCRE2  provides  a  feature  called "callout", which is a means of tem-
       porarily passing control to the caller of PCRE2 in the middle  of  pat-
       tern  matching.  The  caller  of PCRE2 provides an external function by
       putting its entry point in a match context (see pcre2_set_callout()  in
       the pcre2api documentation).

       When  using the pcre2_substitute() function, an additional callout fea-
       ture is available. This does a callout after each change to the subject
       string and is described in the pcre2api documentation; the rest of this
       document is concerned with callouts during pattern matching.

       Within a regular expression, (?C<arg>) indicates a point at  which  the
       external  function  is  to  be  called. Different callout points can be
       identified by putting a number less than 256 after the  letter  C.  The
       default  value is zero.  Alternatively, the argument may be a delimited
       string. The starting delimiter must be one of ` ' " ^ % # $ {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing  delimiter  is  }.  If  the  ending  delimiter is needed within the
       string, it must be doubled. For example, this pattern has  two  callout
       points:

         (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE2  automatically inserts callouts, all with number 255, before each
       item in the pattern except for immediately before or after an  explicit
       callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern

         A(?C3)B

       it is processed as if it were

         (?C255)A(?C3)B(?C255)

       Here is a more complicated example:

         A(\d{2}|--)

       With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were

         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice  that  there  is a callout before and after each parenthesis and
       alternation bar. If the pattern contains a conditional group whose con-
       dition is an assertion, an automatic callout  is  inserted  immediately
       before  the  condition. Such a callout may also be inserted explicitly,
       for example:

         (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

       This applies only to assertion conditions (because they are  themselves
       independent groups).

       Callouts  can  be useful for tracking the progress of pattern matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic callouts.  When any callouts are  present,  the  output  from
       pcre2test  indicates  how  the pattern is being matched. This is useful
       information when you are trying to optimize the performance of  a  par-
       ticular pattern.


MISSING CALLOUTS

       You  should  be  aware  that, because of optimizations in the way PCRE2
       compiles and matches patterns, callouts sometimes do not happen exactly
       as you might expect.

   Auto-possessification

       At compile time, PCRE2 "auto-possessifies" repeated items when it knows
       that what follows cannot be part of the repeat. For example, a+[bc]  is
       compiled  as if it were a++[bc]. The pcre2test output when this pattern
       is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
         No match

       This indicates that when matching [bc] fails, there is no  backtracking
       into a+ (because it is being treated as a++) and therefore the callouts
       that  would  be  taken for the backtracks do not occur. You can disable
       the  auto-possessify  feature  by  passing   PCRE2_NO_AUTO_POSSESS   to
       pcre2_compile(),  or  starting  the pattern with (*NO_AUTO_POSSESS). In
       this case, the output changes to this:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
          +2 ^  ^     [bc]
          +2 ^ ^      [bc]
          +2 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

   Automatic .* anchoring

       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the  dot  can  match
       any  character,  the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after an internal newline or at  the
       beginning of the subject, and pcre2_compile() remembers this. If a pat-
       tern  has more than one top-level branch, automatic anchoring occurs if
       all branches are anchorable.

       This optimization is disabled, however, if .* is in an atomic group  or
       if  there  is a backreference to the capture group in which it appears.
       It is also disabled if the pattern contains (*PRUNE) or  (*SKIP).  How-
       ever, the presence of callouts does not affect it.

       For  example,  if  the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
         No match

       This shows that all match attempts start at the beginning of  the  sub-
       ject. In other words, the pattern is anchored. You can disable this op-
       timization  by  passing  PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
       starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the  out-
       put changes to:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
          +0  ^     .*
          +2  ^^    \d
          +2  ^     \d
         No match

       This  shows more match attempts, starting at the second subject charac-
       ter.  Another optimization, described in the next section,  means  that
       there is no subsequent attempt to match with an empty subject.

   Other optimizations

       Other  optimizations  that  provide fast "no match" results also affect
       callouts.  For example, if the pattern is

         ab(?C4)cd

       PCRE2 knows that any matching string must contain the  letter  "d".  If
       the  subject  string  is  "abyz",  the  lack of "d" means that matching
       doesn't ever start, and the callout is  never  reached.  However,  with
       "abyd", though the result is still no match, the callout is obeyed.

       For  most  patterns  PCRE2  also knows the minimum length of a matching
       string, and will immediately give a "no match" return without  actually
       running  a  match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
       MIZE option  to  pcre2_compile(),  or  by  starting  the  pattern  with
       (*NO_START_OPT).  This slows down the matching process, but does ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During matching, when PCRE2 reaches a callout  point,  if  an  external
       function  is  provided in the match context, it is called. This applies
       to both normal, DFA, and JIT matching. The first argument to the  call-
       out function is a pointer to a pcre2_callout block. The second argument
       is  the  void * callout data that was supplied when the callout was set
       up by calling pcre2_set_callout() (see the pcre2api documentation). The
       callout block structure contains the following fields, not  necessarily
       in this order:

         uint32_t      version;
         uint32_t      callout_number;
         uint32_t      capture_top;
         uint32_t      capture_last;
         uint32_t      callout_flags;
         PCRE2_SIZE   *offset_vector;
         PCRE2_SPTR    mark;
         PCRE2_SPTR    subject;
         PCRE2_SIZE    subject_length;
         PCRE2_SIZE    start_match;
         PCRE2_SIZE    current_position;
         PCRE2_SIZE    pattern_position;
         PCRE2_SIZE    next_item_length;
         PCRE2_SIZE    callout_string_offset;
         PCRE2_SIZE    callout_string_length;
         PCRE2_SPTR    callout_string;

       The  version field contains the version number of the block format. The
       current version is 2; the three callout string fields  were  added  for
       version  1, and the callout_flags field for version 2. If you are writ-
       ing an application that might use an  earlier  release  of  PCRE2,  you
       should  check  the version number before accessing any of these fields.
       The version number will increase in future if more  fields  are  added,
       but the intention is never to remove any of the existing fields.

   Fields for numerical callouts

       For  a  numerical  callout,  callout_string is NULL, and callout_number
       contains the number of the callout, in the range  0-255.  This  is  the
       number  that  follows  (?C for callouts that part of the pattern; it is
       255 for automatically generated callouts.

   Fields for string callouts

       For callouts with string arguments, callout_number is always zero,  and
       callout_string  points  to the string that is contained within the com-
       piled pattern. Its length is given by callout_string_length. Duplicated
       ending delimiters that were present in the original pattern string have
       been turned into single characters, but there is no other processing of
       the callout string argument. An additional code unit containing  binary
       zero  is  present  after the string, but is not included in the length.
       The delimiter that was used to start the string is also  stored  within
       the  pattern, immediately before the string itself. You can access this
       delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit offset to the start of
       the callout argument string within the original pattern string. This is
       provided for the benefit of applications such as script languages  that
       might need to report errors in the callout string within the pattern.

   Fields for all callouts

       The  remaining  fields in the callout block are the same for both kinds
       of callout.

       The offset_vector field is a pointer to a vector of  capturing  offsets
       (the "ovector"). You may read the elements in this vector, but you must
       not change any of them.

       For  calls  to pcre2_match(), the offset_vector field is not (since re-
       lease 10.30) a pointer to the actual ovector that  was  passed  to  the
       matching  function in the match data block. Instead it points to an in-
       ternal ovector of a size large enough to  hold  all  possible  captured
       substrings in the pattern. Note that whenever a recursion or subroutine
       call  within  a pattern completes, the capturing state is reset to what
       it was before.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring,  and the capture_top field contains one more than the
       number of the highest numbered captured substring so far.  If  no  sub-
       strings  have yet been captured, the value of capture_last is 0 and the
       value of capture_top is 1. The values of these  fields  do  not  always
       differ   by   one;  for  example,  when  the  callout  in  the  pattern
       ((a)(b))(?C2) is taken, capture_last is 1 but capture_top is 4.

       The contents of ovector[2] to  ovector[<capture_top>*2-1]  can  be  in-
       spected  in  order to extract substrings that have been matched so far,
       in the same way as extracting substrings after a match  has  completed.
       The  values in ovector[0] and ovector[1] are always PCRE2_UNSET because
       the match is by definition not complete. Substrings that have not  been
       captured  but whose numbers are less than capture_top also have both of
       their ovector slots set to PCRE2_UNSET.

       For DFA matching, the offset_vector field points to  the  ovector  that
       was  passed  to the matching function in the match data block for call-
       outs at the top level, but to an internal ovector during the processing
       of pattern recursions, lookarounds, and atomic groups.  However,  these
       ovectors  hold no useful information because pcre2_dfa_match() does not
       support substring capturing. The value of capture_top is always  1  and
       the value of capture_last is always 0 for DFA matching.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The  start_match  field normally contains the offset within the subject
       at which the current match attempt started. However, if the escape  se-
       quence  \K  has  been encountered, this value is changed to reflect the
       modified starting point. If the pattern is not  anchored,  the  callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       The pattern_position field contains the offset in the pattern string to
       the next item to be matched.

       The next_item_length field contains the length of the next item  to  be
       processed  in the pattern string. When the callout is at the end of the
       pattern, the length is zero.  When  the  callout  precedes  an  opening
       parenthesis, the length includes meta characters that follow the paren-
       thesis.  For  example,  in a callout before an assertion such as (?=ab)
       the length is 3. For an alternation bar or a closing  parenthesis,  the
       length  is  one,  unless a closing parenthesis is followed by a quanti-
       fier, in which case its length is included. (This  changed  in  release
       10.23.  In  earlier  releases, before an opening parenthesis the length
       was that of the entire group, and before an alternation bar or a  clos-
       ing parenthesis the length was zero.)

       The  pattern_position  and next_item_length fields are intended to help
       in distinguishing between different automatic callouts, which all  have
       the  same  callout  number. However, they are set for all callouts, and
       are used by pcre2test to show the next item to be matched when display-
       ing callout information.

       In callouts from pcre2_match() the mark field contains a pointer to the
       zero-terminated name of the most recently passed (*MARK), (*PRUNE),  or
       (*THEN)  item  in the match, or NULL if no such items have been passed.
       Instances of (*PRUNE) or (*THEN) without a name  do  not  obliterate  a
       previous (*MARK). In callouts from the DFA matching function this field
       always contains NULL.

       The   callout_flags   field   is   always   zero   in   callouts   from
       pcre2_dfa_match() or when JIT is being used. When pcre2_match() without
       JIT is used, the following bits may be set:

         PCRE2_CALLOUT_STARTMATCH

       This is set for the first callout after the start of matching for  each
       new starting position in the subject.

         PCRE2_CALLOUT_BACKTRACK

       This  is  set if there has been a matching backtrack since the previous
       callout, or since the start of matching if this is  the  first  callout
       from a pcre2_match() run.

       Both  bits  are  set when a backtrack has caused a "bumpalong" to a new
       starting position in the subject. Output from pcre2test does not  indi-
       cate  the  presence  of these bits unless the callout_extra modifier is
       set.

       The information in the callout_flags field is provided so that applica-
       tions can track and tell their users how matching with backtracking  is
       done.  This  can be useful when trying to optimize patterns, or just to
       understand how PCRE2 works. There is no  support  in  pcre2_dfa_match()
       because  there is no backtracking in DFA matching, and there is no sup-
       port in JIT because JIT is all about maximimizing matching performance.
       In both these cases the callout_flags field is always zero.


RETURN VALUES FROM CALLOUTS

       The external callout function returns an integer to PCRE2. If the value
       is zero, matching proceeds as normal. If  the  value  is  greater  than
       zero,  matching  fails  at  the current point, but the testing of other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the match is abandoned, and the
       matching function returns the negative value.

       Negative values should normally be chosen from  the  set  of  PCRE2_ER-
       ROR_xxx  values.  In  particular, PCRE2_ERROR_NOMATCH forces a standard
       "no match" failure. The error number  PCRE2_ERROR_CALLOUT  is  reserved
       for use by callout functions; it will never be used by PCRE2 itself.


CALLOUT ENUMERATION

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might  like  to  scan  all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the  second  points  to  a
       callback  function,  and the third is arbitrary user data. The callback
       function is called for every callout in the pattern  in  the  order  in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation  block,  and  its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The data block contains  the  fol-
       lowing fields:

         version                Block version number
         pattern_position       Offset to next item in pattern
         next_item_length       Length of next item in pattern
         callout_number         Number for numbered callouts
         callout_string_offset  Offset to string within pattern
         callout_string_length  Length of callout string
         callout_string         Points to callout string or is NULL

       The  version  number is currently 0. It will increase if new fields are
       ever added to the block. The remaining fields are  the  same  as  their
       namesakes  in  the pcre2_callout block that is used for callouts during
       matching, as described above.

       Note that the value of pattern_position is  unique  for  each  callout.
       However,  if  a callout occurs inside a group that is quantified with a
       non-zero minimum or a fixed maximum, the group is replicated inside the
       compiled pattern. For example, a pattern such as /(a){2}/  is  compiled
       as  if it were /(a)(a)/. This means that the callout will be enumerated
       more than once, but with the same value for  pattern_position  in  each
       case.

       The callback function should normally return zero. If it returns a non-
       zero value, scanning the pattern stops, and that value is returned from
       pcre2_callout_enumerate().


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 19 January 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.43                     19 January 2024                PCRE2CALLOUT(3)
------------------------------------------------------------------------------



PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


DIFFERENCES BETWEEN PCRE2 AND PERL

       This  document describes some of the known differences in the ways that
       PCRE2 and Perl handle regular expressions.  The  differences  described
       here  are  with  respect  to  Perl version 5.38.0, but as both Perl and
       PCRE2 are continually changing, the information may at times be out  of
       date.

       1.  When  PCRE2_DOTALL  (equivalent to Perl's /s qualifier) is not set,
       the behaviour of the '.' metacharacter differs from Perl. In PCRE2, '.'
       matches the next character unless it is the  start  of  a  newline  se-
       quence.  This  means  that, if the newline setting is CR, CRLF, or NUL,
       '.' will match the code point LF (0x0A) in ASCII/Unicode  environments,
       and  NL  (either  0x15 or 0x25) when using EBCDIC. In Perl, '.' appears
       never to match LF, even when 0x0A is not a newline indicator.

       2. PCRE2 has only a subset of Perl's Unicode support. Details  of  what
       it does have are given in the pcre2unicode page.

       3.  Like  Perl, PCRE2 allows repeat quantifiers on parenthesized asser-
       tions, but they do not mean what you might think. For example, (?!a){3}
       does not assert that the next three characters are not "a". It just as-
       serts that the next character is not "a"  three  times  (in  principle;
       PCRE2  optimizes this to run the assertion just once). Perl allows some
       repeat quantifiers on other assertions, for example, \b* , but these do
       not seem to have any use. PCRE2 does not allow any kind  of  quantifier
       on non-lookaround assertions.

       4.  If a braced quantifier such as {1,2} appears where there is nothing
       to repeat (for example, at the start of a branch), PCRE2 raises an  er-
       ror whereas Perl treats the quantifier characters as literal.

       5.  Capture groups that occur inside negative lookaround assertions are
       counted, but their entries in the offsets vector are set  only  when  a
       negative  assertion is a condition that has a matching branch (that is,
       the condition is false).  Perl may set such  capture  groups  in  other
       circumstances.

       6.  The  following Perl escape sequences are not supported: \F, \l, \L,
       \u, \U, and \N when followed by a character name. \N on its own, match-
       ing a non-newline character, and \N{U+dd..}, matching  a  Unicode  code
       point,  are  supported.  The  escapes that modify the case of following
       letters are implemented by Perl's general string-handling and  are  not
       part of its pattern matching engine. If any of these are encountered by
       PCRE2,  an  error  is  generated  by default. However, if either of the
       PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U  and  \u  are
       interpreted as ECMAScript interprets them.

       7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
       is built with Unicode support (the default). The properties that can be
       tested  with  \p  and \P are limited to the general category properties
       such as Lu and Nd, the derived properties  Any  and  LC  (synonym  L&),
       script  names such as Greek or Han, Bidi_Class, Bidi_Control, and a few
       binary properties. Both PCRE2 and Perl support the Cs (surrogate) prop-
       erty, but in PCRE2 its use is limited. See the pcre2pattern  documenta-
       tion  for  details. The long synonyms for property names that Perl sup-
       ports (such as \p{Letter}) are not supported by PCRE2, nor is  it  per-
       mitted to prefix any of these properties with "Is".

       8. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
       in between are treated as literals. However, this is slightly different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (PCRE2 does not have
       variables). Also, Perl does "double-quotish backslash interpolation" on
       any backslashes between \Q and \E which, its documentation  says,  "may
       lead  to confusing results". PCRE2 treats a backslash between \Q and \E
       just like any other character. Note the following examples:

           Pattern            PCRE2 matches     Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
           \QA\B\E            A\B               A\B
           \Q\\E              \                 \\E

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes by both PCRE2 and Perl.

       9.   Fairly  obviously,  PCRE2  does  not  support  the  (?{code})  and
       (??{code}) constructions. However, PCRE2 does have a "callout" feature,
       which allows an external function to be called during pattern matching.
       See the pcre2callout documentation for details.

       10. Subroutine calls (whether recursive or not) were treated as  atomic
       groups  up to PCRE2 release 10.23, but from release 10.30 this changed,
       and backtracking into subroutine calls is now supported, as in Perl.

       11. In PCRE2, if any of the backtracking control verbs are  used  in  a
       group  that  is  called  as  a subroutine (whether or not recursively),
       their effect is confined to that group; it does not extend to the  sur-
       rounding  pattern.  This is not always the case in Perl. In particular,
       if (*THEN) is present in a group that is called as  a  subroutine,  its
       action is limited to that group, even if the group does not contain any
       |  characters.  Note  that such groups are processed as anchored at the
       point where they are tested.

       12. If a pattern contains more than one backtracking control verb,  the
       first  one  that  is backtracked onto acts. For example, in the pattern
       A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but  a  failure
       in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
       it is the same as PCRE2, but there are cases where it differs.

       13.  There are some differences that are concerned with the settings of
       captured strings when part of  a  pattern  is  repeated.  For  example,
       matching  "aba"  against the pattern /^(a(b)?)+$/ in Perl leaves $2 un-
       set, but in PCRE2 it is set to "b".

       14. PCRE2's handling of duplicate capture group numbers  and  names  is
       not  as  general as Perl's. This is a consequence of the fact the PCRE2
       works internally just with numbers, using an external table  to  trans-
       late  between  numbers  and  names.  In  particular,  a pattern such as
       (?|(?<a>A)|(?<b>B)), where the two capture groups have the same  number
       but  different  names, is not supported, and causes an error at compile
       time. If it were allowed, it would not be possible to distinguish which
       group matched, because both names map to capture  group  number  1.  To
       avoid this confusing situation, an error is given at compile time.

       15. Perl used to recognize comments in some places that PCRE2 does not,
       for  example,  between  the  ( and ? at the start of a group. If the /x
       modifier is set, Perl allowed white space between ( and  ?  though  the
       latest  Perls give an error (for a while it was just deprecated). There
       may still be some cases where Perl behaves differently.

       16. Perl, when in warning mode, gives warnings  for  character  classes
       such  as  [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter-
       als. PCRE2 has no warning features, so it gives an error in these cases
       because they are almost certainly user mistakes.

       17. In PCRE2, the upper/lower case character properties Lu and  Ll  are
       not  affected when case-independent matching is specified. For example,
       \p{Lu} always matches an upper case letter. I think Perl has changed in
       this respect; in the release at the time of writing (5.38), \p{Lu}  and
       \p{Ll} match all letters, regardless of case, when case independence is
       specified.

       18. From release 5.32.0, Perl locks out the use of \K in lookaround as-
       sertions.  From  release 10.38 PCRE2 does the same by default. However,
       there is an option for re-enabling the previous  behaviour.  When  this
       option  is  set,  \K is acted on when it occurs in positive assertions,
       but is ignored in negative assertions.

       19. PCRE2 provides some extensions to the Perl regular  expression  fa-
       cilities.   Perl  5.10  included  new features that were not in earlier
       versions of Perl, some of which (such as  named  parentheses)  were  in
       PCRE2 for some time before. This list is with respect to Perl 5.38:

       (a)  If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
       $ meta-character matches only at the very end of the string.

       (b) A backslash followed  by  a  letter  with  no  special  meaning  is
       faulted. (Perl can be made to issue a warning.)

       (c)  If PCRE2_UNGREEDY is set, the greediness of the repetition quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (d) PCRE2_ANCHORED can be used at matching time to force a  pattern  to
       be tried only at the first matching position in the subject string.

       (e)     The     PCRE2_NOTBOL,    PCRE2_NOTEOL,    PCRE2_NOTEMPTY    and
       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.

       (f) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE2_BSR_ANYCRLF option.

       (g)  The  callout  facility is PCRE2-specific. Perl supports codeblocks
       and variable interpolation, but not general hooks on every match.

       (h) The partial matching facility is PCRE2-specific.

       (i) The alternative matching function (pcre2_dfa_match() matches  in  a
       different way and is not Perl-compatible.

       (j)  PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
       at the start of a pattern. These set overall  options  that  cannot  be
       changed within the pattern.

       (k)  PCRE2  supports non-atomic positive lookaround assertions. This is
       an extension to the lookaround facilities. The default, Perl-compatible
       lookarounds are atomic.

       (l) There are three syntactical items in patterns that can refer  to  a
       capturing  group  by  number: back references such as \g{2}, subroutine
       calls such as (?3), and condition references such as  (?(4)...).  PCRE2
       supports  relative  group numbers such as +2 and -4 in all three cases.
       Perl supports both plus and minus for subroutine calls, but only  minus
       for back references, and no relative numbering at all for conditions.

       20. Perl has different limits than PCRE2. See the pcre2limit documenta-
       tion for details. Perl went with 5.10 from recursion to iteration keep-
       ing the intermediate matches on the heap, which is ~10% slower but does
       not  fall into any stack-overflow limit. PCRE2 made a similar change at
       release 10.30, and also has many build-time and  run-time  customizable
       limits.

       21.  Unlike  Perl,  PCRE2 doesn't have character set modifiers and spe-
       cially no way to set characters by context just  like  Perl's  "/d".  A
       regular expression using PCRE2_UTF and PCRE2_UCP will use similar rules
       to  Perl's  "/u";  something closer to "/a" could be selected by adding
       other PCRE2_EXTRA_ASCII* options on top.

       22. Some recursive patterns that Perl diagnoses as infinite  recursions
       can be handled by PCRE2, either by the interpreter or the JIT. An exam-
       ple is /(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of any number
       of repeated "abcd" substrings at the end of the subject.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 30 November 2023
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.43                    30 November 2023                 PCRE2COMPAT(3)
------------------------------------------------------------------------------



PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing  before  the  match is performed, so it is of most benefit when
       the same pattern is going to be matched many times. This does not  nec-
       essarily  mean many calls of a matching function; if the pattern is not
       anchored, matching attempts may take place many times at various  posi-
       tions in the subject, even for a single call. Therefore, if the subject
       string  is  very  long,  it  may  still pay to use JIT even for one-off
       matches. JIT support is available for all  of  the  8-bit,  16-bit  and
       32-bit PCRE2 libraries.

       JIT  support  applies  only to the traditional Perl-compatible matching
       function.  It does not apply when the DFA matching  function  is  being
       used. The code for JIT support was written by Zoltan Herczeg.


AVAILABILITY OF JIT SUPPORT

       JIT  support  is  an  optional feature of PCRE2. The "configure" option
       --enable-jit (or equivalent CMake option) must be  set  when  PCRE2  is
       built  if  you want to use JIT. The support is limited to the following
       hardware platforms:

         ARM 32-bit (v7, and Thumb2)
         ARM 64-bit
         IBM s390x 64 bit
         Intel x86 32-bit and 64-bit
         LoongArch 64 bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         RISC-V 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A client program can tell  if  JIT  support  is  available  by  calling
       pcre2_config()  with  the PCRE2_CONFIG_JIT option. The result is one if
       PCRE2 was built with JIT support, and zero otherwise.  However,  having
       the  JIT code available does not guarantee that it will be used for any
       particular match. One reason for this is that there are a number of op-
       tions and pattern items that are not supported by JIT (see below).  An-
       other  reason  is that in some environments JIT is unable to get memory
       in which to build its compiled code. The only guarantee from pcre2_con-
       fig() is that if it returns zero, JIT will definitely not be used.

       A simple program does not need to check availability in  order  to  use
       JIT  when  possible. The API is implemented in a way that falls back to
       the interpretive code if JIT is not available or cannot be used  for  a
       given  match.  For  programs  that  need the best possible performance,
       there is a "fast path" API that is JIT-specific.


SIMPLE USE OF JIT

       To make use of the JIT support in the simplest way, all you have to  do
       is  to  call pcre2_jit_compile() after successfully compiling a pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled pattern pointer that was returned by pcre2_compile(), and  the
       second  is  zero  or  more of the following option bits: PCRE2_JIT_COM-
       PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       If JIT support is not available, a  call  to  pcre2_jit_compile()  does
       nothing  and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
       pattern is passed to the JIT compiler, which turns it into machine code
       that executes much faster than the normal interpretive code, but yields
       exactly the same results. The returned value  from  pcre2_jit_compile()
       is zero on success, or a negative error code.

       There  is  a limit to the size of pattern that JIT supports, imposed by
       the size of machine stack that it uses. The exact rules are  not  docu-
       mented because they may change at any time, in particular, when new op-
       timizations  are  introduced.   If  a  pattern  is  too  big, a call to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for  com-
       plete  matches. If you want to run partial matches using the PCRE2_PAR-
       TIAL_HARD or PCRE2_PARTIAL_SOFT options of  pcre2_match(),  you  should
       set  one  or  both  of  the  other  options  as  well as, or instead of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for each of the three modes (normal, soft partial, hard partial).  When
       pcre2_match()  is  called,  the appropriate code is run if it is avail-
       able. Otherwise, the pattern is matched using interpretive code.

       You can call pcre2_jit_compile() multiple times for the  same  compiled
       pattern.  It does nothing if it has previously compiled code for any of
       the option bits. For example, you can call it once with  PCRE2_JIT_COM-
       PLETE  and  (perhaps  later,  when  you find you need partial matching)
       again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time  it
       will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
       ing. If pcre2_jit_compile() is called with no option bits set, it imme-
       diately returns zero. This is an alternative way of testing whether JIT
       is available.

       At  present,  it  is not possible to free JIT compiled code except when
       the entire compiled pattern is freed by calling pcre2_code_free().

       In some circumstances you may need to call additional functions.  These
       are  described  in the section entitled "Controlling the JIT stack" be-
       low.

       There are some pcre2_match() options that are not supported by JIT, and
       there are also some pattern items that JIT cannot handle.  Details  are
       given  below.   In both cases, matching automatically falls back to the
       interpretive code. If you want to know whether JIT  was  actually  used
       for  a particular match, you should arrange for a JIT callback function
       to be set up as described in the section entitled "Controlling the  JIT
       stack"  below,  even  if  you  do  not need to supply a non-default JIT
       stack. Such a callback function is called whenever JIT code is about to
       be obeyed. If the match-time options are not right for  JIT  execution,
       the callback function is not obeyed.

       If  the  JIT  compiler finds an unsupported item, no JIT data is gener-
       ated. You can find out if JIT compilation was successful for a compiled
       pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE op-
       tion. A non-zero result means that JIT compilation  was  successful.  A
       result of 0 means that JIT support is not available, or the pattern was
       not  processed by pcre2_jit_compile(), or the JIT compiler was not able
       to handle the pattern. Successful JIT compilation  does  not,  however,
       guarantee  the  use  of  JIT at match time because there are some match
       time options that are not supported by JIT.


MATCHING SUBJECTS CONTAINING INVALID UTF

       When a pattern is compiled with the PCRE2_UTF option,  subject  strings
       are  normally expected to be a valid sequence of UTF code units. By de-
       fault, this is checked at the start of matching and an error is  gener-
       ated  if  invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can be
       passed to pcre2_match() to skip the check (for improved performance) if
       you are sure that a subject string is valid. If  this  option  is  used
       with  an  invalid  string, the result is undefined. The calling program
       may crash or loop or otherwise misbehave.

       However, a way of running matches on strings that may  contain  invalid
       UTF   sequences   is   available.   Calling  pcre2_compile()  with  the
       PCRE2_MATCH_INVALID_UTF option has two effects:  it  tells  the  inter-
       preter  in pcre2_match() to support invalid UTF, and, if pcre2_jit_com-
       pile() is subsequently called, the compiled JIT code also supports  in-
       valid  UTF.  Details of how this support works, in both the JIT and the
       interpretive cases, is given in the pcre2unicode documentation.

       There  is  also  an  obsolete  option  for  pcre2_jit_compile()  called
       PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat-
       ibility.     It   is   superseded   by   the   pcre2_compile()   option
       PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed
       in future.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The pcre2_match() options that  are  supported  for  JIT  matching  are
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,   PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,  and
       PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED  and  PCRE2_ENDANCHORED  options
       are not supported at match time.

       If  the  PCRE2_NO_JIT option is passed to pcre2_match() it disables the
       use of JIT, forcing matching by the interpreter code.

       The only unsupported pattern items are \C (match a  single  data  unit)
       when  running in a UTF mode, and a callout immediately before an asser-
       tion condition in a conditional group.


RETURN VALUES FROM JIT MATCHING

       When a pattern is matched using JIT, the return values are the same  as
       those  given  by the interpretive pcre2_match() code, with the addition
       of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that  the
       memory  used  for  the JIT stack was insufficient. See "Controlling the
       JIT stack" below for a discussion of JIT stack usage.

       The error code PCRE2_ERROR_MATCHLIMIT is returned by the  JIT  code  if
       searching  a  very large pattern tree goes on for too long, as it is in
       the same circumstance when JIT is not used, but the details of  exactly
       what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code
       is never returned when JIT matching is used.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.   By  default, it uses 32KiB on the machine stack. However, some
       large or complicated patterns need more than this. The error  PCRE2_ER-
       ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func-
       tions are provided for managing blocks of memory for use as JIT stacks.
       There  is further discussion about the use of JIT stacks in the section
       entitled "JIT stack FAQ" below.

       The pcre2_jit_stack_create() function creates a JIT  stack.  Its  argu-
       ments  are  a starting size, a maximum size, and a general context (for
       memory allocation functions, or NULL for standard  memory  allocation).
       It returns a pointer to an opaque structure of type pcre2_jit_stack, or
       NULL  if there is an error. The pcre2_jit_stack_free() function is used
       to free a stack that is no longer needed. If its argument is NULL, this
       function returns immediately, without doing anything. (For the  techni-
       cally  minded: the address space is allocated by mmap or VirtualAlloc.)
       A maximum stack size of 512KiB to 1MiB should be more than  enough  for
       any pattern.

       The  pcre2_jit_stack_assign()  function  specifies which stack JIT code
       should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is subse-
       quently passed to a matching function, its information determines which
       JIT stack is used. If this argument is NULL, the function returns imme-
       diately, without doing anything. There are three cases for  the  values
       of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32KiB block
             on the machine stack is used. This is the default when a match
             context is created.

         (2) If callback is NULL and data is not NULL, data must be
             a pointer to a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32KiB stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

       A  callback function is obeyed whenever JIT code is about to be run; it
       is not obeyed when pcre2_match() is called with options that are incom-
       patible for JIT matching. A callback function can therefore be used  to
       determine  whether  a match operation was executed by JIT or by the in-
       terpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by assigning directly or by callback), as  long  as  the  patterns  are
       matched sequentially in the same thread. Currently, the only way to set
       up  non-sequential matches in one thread is to use callouts: if a call-
       out function starts another match, that match must use a different  JIT
       stack to the one used for currently suspended match(es).

       In  a multithread application, if you do not specify a JIT stack, or if
       you assign or pass back NULL from a callback, that is thread-safe,  be-
       cause  each thread has its own machine stack. However, if you assign or
       pass back a non-NULL JIT stack, this must be a different stack for each
       thread so that the application is thread-safe.

       Strictly speaking, even more is allowed. You can assign the  same  non-
       NULL  stack  to a match context that is used by any number of patterns,
       as long as they are not used for matching by multiple  threads  at  the
       same  time.  For  example, you could use the same stack in all compiled
       patterns, with a global mutex in the callback to wait until  the  stack
       is available for use. However, this is an inefficient solution, and not
       recommended.

       This  is a suggestion for how a multithreaded program that needs to set
       up non-default JIT stacks might operate:

         During thread initialization
           thread_local_var = pcre2_jit_stack_create(...)

         During thread exit
           pcre2_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All the functions described in this section do nothing if  JIT  is  not
       available.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
       where  the local data of the current node is pushed before checking its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend the stack on PowerPC.  Although it  is  possible,  its  updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern  operating  systems have a nice feature: they can reserve an ad-
       dress space instead of allocating memory. We can safely allocate memory
       pages inside this address space, so the stack could grow without moving
       memory data (this is important because of pointers). Thus we can  allo-
       cate  1MiB  address  space,  and use only a single memory page (usually
       4KiB) if that is enough. However, we can still grow up to 1MiB  anytime
       if needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is  passed  to  the  pattern currently running), that stack must not be
       used by any other threads (to avoid overwriting the same memory  area).
       The best practice for multithreaded programs is to allocate a stack for
       each thread, and return this stack through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre2_match() again. When you assign the stack to a match context, only
       a  pointer  is  set. There is no reference counting or any other magic.
       You can free compiled patterns, contexts, and stacks in any order, any-
       time.  Just do not call pcre2_match() with a match context pointing  to
       an already freed stack, as that will cause SEGFAULT. (Also, do not free
       a  stack  currently  used  by pcre2_match() in another thread). You can
       also replace the stack in a context at any time when it is not in  use.
       You should free the previous stack before assigning a replacement.

       (5)  Should  I  allocate/free  a  stack every time before/after calling
       pcre2_match()?

       No, because this is too costly in  terms  of  resources.  However,  you
       could  implement  some clever idea which release the stack if it is not
       used in let's say two minutes. The JIT callback  can  help  to  achieve
       this without keeping a list of patterns.

       (6)  OK, the stack is for long term memory allocation. But what happens
       if a pattern causes stack overflow with a stack of 1MiB? Is  that  1MiB
       kept until the stack is freed?

       Especially on embedded systems, it might be a good idea to release mem-
       ory  sometimes  without  freeing the stack. There is no API for this at
       the moment.  Probably a function call which returns with the  currently
       allocated  memory for any stack and another which allows releasing mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No, thanks to Windows. If POSIX threads were used everywhere, we  could
       throw out this complicated API.


FREEING JIT SPECULATIVE MEMORY

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is possi-
       ble.  It  expects new allocations, and keeps some free memory around to
       improve allocation speed. However, in low memory conditions,  it  might
       be  better to free all possible memory. You can cause this to happen by
       calling pcre2_jit_free_unused_memory(). Its argument is a general  con-
       text, for custom memory management, or NULL for standard memory manage-
       ment.


EXAMPLE CODE

       This  is  a  single-threaded example that specifies a JIT stack without
       using a callback. A real program should include  error  checking  after
       all the function calls.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
           &errornumber, &erroffset, NULL);
         rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
         mcontext = pcre2_match_context_create(NULL);
         jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
         pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
         match_data = pcre2_match_data_create(re, 10);
         rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
         /* Process result */

         pcre2_code_free(re);
         pcre2_match_data_free(match_data);
         pcre2_match_context_free(mcontext);
         pcre2_jit_stack_free(jit_stack);


JIT FAST PATH API

       Because the API described above falls back to interpreted matching when
       JIT  is  not  available, it is convenient for programs that are written
       for  general  use  in  many  environments.  However,  calling  JIT  via
       pcre2_match() does have a performance impact. Programs that are written
       for  use  where  JIT  is known to be available, and which need the best
       possible performance, can instead use a "fast path"  API  to  call  JIT
       matching  directly instead of calling pcre2_match() (obviously only for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The fast path function is called pcre2_jit_match(), and  it  takes  ex-
       actly  the same arguments as pcre2_match(). However, the subject string
       must be specified with a  length;  PCRE2_ZERO_TERMINATED  is  not  sup-
       ported.  Unsupported  option  bits  (for  example,  PCRE2_ANCHORED  and
       PCRE2_ENDANCHORED) are ignored, as is the PCRE2_NO_JIT option. The  re-
       turn  values  are  also  the  same as for pcre2_match(), plus PCRE2_ER-
       ROR_JIT_BADOPTION if a matching mode (partial or complete) is requested
       that was not compiled.

       When you call pcre2_match(), as well as testing for invalid options,  a
       number of other sanity checks are performed on the arguments. For exam-
       ple,  if the subject pointer is NULL but the length is non-zero, an im-
       mediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set,  a  UTF
       subject string is tested for validity. In the interests of speed, these
       checks  do  not  happen  on  the  JIT fast path. If invalid UTF data is
       passed when PCRE2_MATCH_INVALID_UTF was not  set  for  pcre2_compile(),
       the  result  is  undefined. The program may crash or loop or give wrong
       results. In the absence  of  PCRE2_MATCH_INVALID_UTF  you  should  call
       pcre2_jit_match()  in  UTF  mode  only  if  you are sure the subject is
       valid.

       Bypassing the sanity checks and the  pcre2_match()  wrapping  can  give
       speedups of more than 10%.


SEE ALSO

       pcre2api(3), pcre2unicode(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 21 February 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.43                    21 February 2024                    PCRE2JIT(3)
------------------------------------------------------------------------------



PCRE2LIMITS(3)             Library Functions Manual             PCRE2LIMITS(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SIZE AND OTHER LIMITATIONS

       There are some size limitations in PCRE2 but it is hoped that they will
       never in practice be relevant.

       The  maximum  size  of  a compiled pattern is approximately 64 thousand
       code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with
       the default internal linkage size, which  is  2  bytes  for  these  li-
       braries.  If  you  want  to  process regular expressions that are truly
       enormous, you can compile PCRE2 with an internal linkage size of 3 or 4
       (when building the 16-bit library, 3 is  rounded  up  to  4).  See  the
       README file in the source distribution and the pcre2build documentation
       for  details.  In  these cases the limit is substantially larger.  How-
       ever, the speed of execution is slower. In the 32-bit library, the  in-
       ternal linkage size is always 4.

       The maximum length of a source pattern string is essentially unlimited;
       it  is  the largest number a PCRE2_SIZE variable can hold. However, the
       program that calls pcre2_compile() can specify a smaller limit.

       The maximum length (in code units) of a subject string is one less than
       the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an un-
       signed integer type, usually defined as size_t. Its maximum value (that
       is ~(PCRE2_SIZE)0) is reserved as a special indicator  for  zero-termi-
       nated strings and unset offsets.

       All values in repeating quantifiers must be less than 65536.

       There are two different limits that apply to branches of lookbehind as-
       sertions.   If every branch in such an assertion matches a fixed number
       of characters, the maximum length of any branch is 65535 characters. If
       any branch matches a variable number of characters,  then  the  maximum
       matching  length  for every branch is limited. The default limit is set
       at compile time, defaulting to 255, but can be changed by  the  calling
       program.

       There  is no limit to the number of parenthesized groups, but there can
       be no more than 65535 capture groups, and there is a limit to the depth
       of nesting of parenthesized subpatterns of all kinds. This  is  imposed
       in  order to limit the amount of system stack used at compile time. The
       default limit can be specified when PCRE2 is built; if not, the default
       is set to  250.  An  application  can  change  this  limit  by  calling
       pcre2_set_parens_nest_limit() to set the limit in a compile context.

       The  maximum length of name for a named capture group is 32 code units,
       and the maximum number of such groups is 10000.

       The maximum length of a  name  in  a  (*MARK),  (*PRUNE),  (*SKIP),  or
       (*THEN)  verb  is  255  code units for the 8-bit library and 65535 code
       units for the 16-bit and 32-bit libraries.

       The maximum length of a string argument to a  callout  is  the  largest
       number a 32-bit unsigned integer can hold.

       The  maximum  amount  of heap memory used for matching is controlled by
       the heap limit, which can be set in a pattern or in  a  match  context.
       The default is a very large number, effectively unlimited.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: August 2023
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.43                      1 August 2023                  PCRE2LIMITS(3)
------------------------------------------------------------------------------



PCRE2MATCHING(3)           Library Functions Manual           PCRE2MATCHING(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in  PCRE2  for  matching  a compiled regular expression against a given
       subject string. The "standard" algorithm is the  one  provided  by  the
       pcre2_match() function. This works in the same as Perl's matching func-
       tion,  and  provide  a Perl-compatible matching operation. The just-in-
       time (JIT) optimization that is described in the pcre2jit documentation
       is compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match() function;
       it operates in a different way, and is not Perl-compatible. This alter-
       native has advantages and disadvantages compared with the standard  al-
       gorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented as a tree structure. An unlimited repetition  in  the  pattern
       makes  the  tree of infinite size, but it is still a tree. Matching the
       pattern to a given subject string (from a given starting point) can  be
       thought  of  as  a  search of the tree.  There are two ways to search a
       tree: depth-first and breadth-first, and these correspond  to  the  two
       matching algorithms provided by PCRE2.


THE STANDARD MATCHING ALGORITHM

       In  the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
       sions", the standard algorithm is an "NFA  algorithm".  It  conducts  a
       depth-first  search  of  the pattern tree. That is, it proceeds along a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the alternations and the greedy or ungreedy repetition quan-
       tifiers are specified in the pattern.

       Because it ends up with a single path through the  tree,  it  is  rela-
       tively  straightforward  for  this  algorithm to keep track of the sub-
       strings that are matched by portions of  the  pattern  in  parentheses.
       This provides support for capturing parentheses and backreferences.


THE ALTERNATIVE MATCHING ALGORITHM

       This  algorithm  conducts  a breadth-first search of the tree. Starting
       from the first matching point in the  subject,  it  scans  the  subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding the current point have to be independently in-
       spected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them,  and  in  particular,  it  finds  the
       longest.  The  matches  are returned in the output vector in decreasing
       order of length. There is an option to stop  the  algorithm  after  the
       first match (which is necessarily the shortest) is found.

       Note  that the size of vector needed to contain all the results depends
       on the number of simultaneous matches, not on the number of parentheses
       in the pattern. Using pcre2_match_data_create_from_pattern() to  create
       the  match  data block is therefore not advisable when doing DFA match-
       ing.

       Note also that all the matches that are found start at the  same  point
       in the subject. If the pattern

         cat(er(pillar)?)?

       is  matched  against the string "the caterpillar catchment", the result
       is the three strings "caterpillar", "cater", and "cat"  that  start  at
       the  fifth  character  of the subject. The algorithm does not automati-
       cally move on to find matches that start at later positions.

       PCRE2's "auto-possessification" optimization usually applies to charac-
       ter repeats at the end of a pattern (as well as internally). For  exam-
       ple, the pattern "a\d+" is compiled as if it were "a\d++" because there
       is  no  point even considering the possibility of backtracking into the
       repeated digits. For DFA matching, this means that  only  one  possible
       match  is  found. If you really do want multiple matches in such cases,
       either use an ungreedy repeat ("a\d+?") or set  the  PCRE2_NO_AUTO_POS-
       SESS option when compiling.

       There  are  a  number of features of PCRE2 regular expressions that are
       not supported or behave differently in the alternative  matching  func-
       tion. Those that are not supported cause an error if encountered.

       1.  Because the algorithm finds all possible matches, the greedy or un-
       greedy nature of repetition quantifiers is not relevant (though it  may
       affect  auto-possessification,  as  just  described).  During matching,
       greedy and ungreedy quantifiers are treated in exactly  the  same  way.
       However, possessive quantifiers can make a difference when what follows
       could  also  match  what  is  quantified, for example in a pattern like
       this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is not straightforward to keep track of  captured  substrings  for  the
       different  matching  possibilities,  and PCRE2's implementation of this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, backreferences within  the  pat-
       tern are not supported.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5. Again for the same reason, script runs are not supported.

       6. Because many paths through the tree may be active, the \K escape se-
       quence,  which  resets the start of the match when encountered (but may
       be on some paths and not on others), is not supported.

       7. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always 0.

       8.  The  \C  escape  sequence, which (in the standard algorithm) always
       matches a single code unit, even in a UTF mode,  is  not  supported  in
       these  modes,  because the alternative algorithm moves through the sub-
       ject string one character (not code unit) at a  time,  for  all  active
       paths through the tree.

       9.  Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
       are not supported. (*FAIL) is supported, and  behaves  like  a  failing
       negative assertion.

       10.  The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not sup-
       ported by pcre2_dfa_match().


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The main advantage of the alternative algorithm is  that  all  possible
       matches (at a single point in the subject) are automatically found, and
       in  particular, the longest match is found. To find more than one match
       at the same point using the standard algorithm, you have to  do  kludgy
       things with callouts.

       Partial  matching  is  possible with this algorithm, though it has some
       limitations. The pcre2partial documentation gives  details  of  partial
       matching and discusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1.  It  is  substantially  slower  than the standard algorithm. This is
       partly because it has to search for all possible matches, but  is  also
       because it is less susceptible to optimization.

       2.  Capturing  parentheses,  backreferences,  script runs, and matching
       within invalid UTF string are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.

       4. JIT optimization is not supported.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 19 January 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.43                     19 January 2024               PCRE2MATCHING(3)
------------------------------------------------------------------------------



PCRE2PARTIAL(3)            Library Functions Manual            PCRE2PARTIAL(3)


NAME
       PCRE2 - Perl-compatible regular expressions


PARTIAL MATCHING IN PCRE2

       In  normal use of PCRE2, if there is a match up to the end of a subject
       string, but more characters are needed to  match  the  entire  pattern,
       PCRE2_ERROR_NOMATCH  is  returned,  just  like any other failing match.
       There are circumstances where it might be helpful to  distinguish  this
       "partial match" case.

       One  example  is  an application where the subject string is very long,
       and not all available at once. The requirement here is to be able to do
       the matching segment by segment, but special action is  needed  when  a
       matched substring spans the boundary between two segments.

       Another  example is checking a user input string as it is typed, to en-
       sure that it conforms to a required format. Invalid characters  can  be
       immediately diagnosed and rejected, giving instant feedback.

       Partial  matching  is a PCRE2-specific feature; it is not Perl-compati-
       ble. It is requested  by  setting  one  of  the  PCRE2_PARTIAL_HARD  or
       PCRE2_PARTIAL_SOFT  options  when calling a matching function. The dif-
       ference between the two options is whether or not a  partial  match  is
       preferred  to  an alternative complete match, though the details differ
       between the two types of matching function. If both  options  are  set,
       PCRE2_PARTIAL_HARD takes precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       as well as setting a partial match option for  the  matching  function,
       you  must  also  call pcre2_jit_compile() with one or both of these op-
       tions:

         PCRE2_JIT_PARTIAL_HARD
         PCRE2_JIT_PARTIAL_SOFT

       PCRE2_JIT_COMPLETE should also be set if you are going to run  non-par-
       tial  matches  on  the same pattern. Separate code is compiled for each
       mode. If the appropriate JIT mode has not been  compiled,  interpretive
       matching code is used.

       Setting  a partial matching option disables two of PCRE2's standard op-
       timization hints. PCRE2 remembers the last literal code unit in a  pat-
       tern,  and  abandons  matching  immediately if it is not present in the
       subject string.  This optimization cannot be used for a subject  string
       that  might match only partially. PCRE2 also remembers a minimum length
       of a matching string, and does not bother to run the matching  function
       on  shorter  strings.  This  optimization  is also disabled for partial
       matching.


REQUIREMENTS FOR A PARTIAL MATCH

       A possible partial match occurs during matching when  the  end  of  the
       subject  string is reached successfully, but either more characters are
       needed to complete the match, or the addition of more characters  might
       change what is matched.

       Example  1: if the pattern is /abc/ and the subject is "ab", more char-
       acters are definitely needed to complete a match.  In  this  case  both
       hard and soft matching options yield a partial match.

       Example  2: if the pattern is /ab+/ and the subject is "ab", a complete
       match can be found, but the addition of more  characters  might  change
       what  is  matched. In this case, only PCRE2_PARTIAL_HARD returns a par-
       tial match; PCRE2_PARTIAL_SOFT returns the complete match.

       On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set,  if
       the next pattern item is \z, \Z, \b, \B, or $ there is always a partial
       match.   Otherwise, for both options, the next pattern item must be one
       that inspects a character, and at least one of the  following  must  be
       true:

       (1)  At  least  one  character has already been inspected. An inspected
       character need not form part of the final  matched  string;  lookbehind
       assertions  and the \K escape sequence provide ways of inspecting char-
       acters before the start of a matched string.

       (2) The pattern contains one or more lookbehind assertions. This condi-
       tion exists in case there is a lookbehind that inspects characters  be-
       fore the start of the match.

       (3)  There  is a special case when the whole pattern can match an empty
       string.  When the starting point is at the  end  of  the  subject,  the
       empty  string  match is a possibility, and if PCRE2_PARTIAL_SOFT is set
       and neither of the above conditions is true, it is  returned.  However,
       because  adding  more  characters  might  result  in a non-empty match,
       PCRE2_PARTIAL_HARD returns a partial match, which in  this  case  means
       "there  is going to be a match at this point, but until some more char-
       acters are added, we do not know if it will be an empty string or some-
       thing longer".


PARTIAL MATCHING USING pcre2_match()

       When  a  partial  matching  option  is  set,  the  result  of   calling
       pcre2_match() can be one of the following:

       A successful match
         A complete match has been found, starting and ending within this sub-
         ject.

       PCRE2_ERROR_NOMATCH
         No match can start anywhere in this subject.

       PCRE2_ERROR_PARTIAL
         Adding  more  characters may result in a complete match that uses one
         or more characters from the end of this subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the pat-
       tern has no effect for a partial match. Consider this pattern:

         /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and the ovector defines the matched string as "123", because \K  resets
       the  "start  of  match" point. However, if a partial match is requested
       and the subject string is "456abc12", a partial match is found for  the
       string  "abc12",  because  all these characters are needed for a subse-
       quent re-match with additional characters.

       If there is more than one partial match, the first one that  was  found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If  this is matched against the subject string "abc123dog", both alter-
       natives fail to match, but the end of the  subject  is  reached  during
       matching,  so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3
       and 9, identifying "123dog" as the first partial match. (In this  exam-
       ple,  there are two partial matches, because "dog" on its own partially
       matches the second alternative.)

   How a partial match is processed by pcre2_match()

       What happens when a partial match is identified depends on which of the
       two partial matching options is set.

       If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned  as  soon
       as  a partial match is found, without continuing to search for possible
       complete matches. This option is "hard" because it prefers  an  earlier
       partial match over a later complete match. For this reason, the assump-
       tion  is  made  that  the end of the supplied subject string is not the
       true end of the available data, which is why \z, \Z, \b, \B, and $  al-
       ways give a partial match.

       If  PCRE2_PARTIAL_SOFT  is  set,  the  partial match is remembered, but
       matching continues as normal, and other alternatives in the pattern are
       tried. If no complete match can be found,  PCRE2_ERROR_PARTIAL  is  re-
       turned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because it
       prefers a complete match over a partial match. All the various matching
       items  in a pattern behave as if the subject string is potentially com-
       plete; \z, \Z, and $ match at the end of the subject,  as  normal,  and
       for \b and \B the end of the subject is treated as a non-alphanumeric.

       The  difference  between the two partial matching options can be illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it  prefers
       the  longer  string  if  possible). If it is matched against the string
       "dog" with PCRE2_PARTIAL_SOFT, it yields a complete  match  for  "dog".
       However,  if  PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR-
       TIAL. On the other hand, if the pattern is made ungreedy the result  is
       different:

         /dog(sbody)??/

       In  this  case  the  result  is always a complete match because that is
       found first, and matching never  continues  after  finding  a  complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The  second pattern will never match "dogsbody", because it will always
       find the shorter match first.

   Example of partial matching using pcre2test

       The pcre2test data modifiers partial_hard (or ph) and partial_soft  (or
       ps)  set  PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when
       calling pcre2_match(). Here is a run of pcre2test using a pattern  that
       matches the whole subject in the form of a date:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25dec3\=ph
         Partial match: 23dec3
         data> 3ju\=ph
         Partial match: 3ju
         data> 3juj\=ph
         No match

       This  example  gives  the  same  results for both hard and soft partial
       matching options. Here is an example where there is a difference:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\=ps
          0: 25jun04
          1: jun
         data> 25jun04\=ph
         Partial match: 25jun04

       With  PCRE2_PARTIAL_SOFT,  the  subject  is  matched  completely.   For
       PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete,
       so there is only a partial match.


MULTI-SEGMENT MATCHING WITH pcre2_match()

       PCRE  was  not originally designed with multi-segment matching in mind.
       However, over time, features (including  partial  matching)  that  make
       multi-segment matching possible have been added. A very long string can
       be  searched  segment  by  segment by calling pcre2_match() repeatedly,
       with the aim of achieving the same results that would happen if the en-
       tire string was available for searching all  the  time.  Normally,  the
       strings  that  are  being  sought are much shorter than each individual
       segment, and are in the middle of very long strings, so the pattern  is
       normally not anchored.

       Special  logic  must  be implemented to handle a matched substring that
       spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it
       returns a partial match at the end of a segment whenever there  is  the
       possibility  of  changing  the  match  by  adding  more characters. The
       PCRE2_NOTBOL option should also be set for all but the first segment.

       When a partial match occurs, the next segment must be added to the cur-
       rent subject and the match re-run, using the  startoffset  argument  of
       pcre2_match()  to  begin  at the point where the partial match started.
       For example:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> ...the date is 23ja\=ph
         Partial match: 23ja
         data> ...the date is 23jan19 and on that day...\=offset=15
          0: 23jan19
          1: jan

       Note the use of the offset modifier to start the new  match  where  the
       partial match was found. In this example, the next segment was added to
       the  one  in  which  the  partial  match  was  found.  This is the most
       straightforward approach, typically using a memory buffer that is twice
       the size of each segment. After a partial match, the first half of  the
       buffer  is  discarded,  the  second  half  is moved to the start of the
       buffer, and a new segment is added before repeating the match as in the
       example above. After a no match, the entire buffer can be discarded.

       If there are memory constraints, you may want to discard text that pre-
       cedes a partial match before adding the  next  segment.  Unfortunately,
       this  is  not  at  present straightforward. In cases such as the above,
       where the pattern does not contain any lookbehinds, it is sufficient to
       retain only the partially matched substring. However,  if  the  pattern
       contains  a  lookbehind assertion, characters that precede the start of
       the partial match may have been inspected during the matching  process.
       When  pcre2test displays a partial match, it indicates these characters
       with '<' if the allusedtext modifier is set:

           re> "(?<=123)abc"
         data> xx123ab\=ph,allusedtext
         Partial match: 123ab
                        <<<

       However, the allusedtext modifier is not available  for  JIT  matching,
       because  JIT  matching  does  not  record the first (or last) consulted
       characters.  For this reason, this information is not available via the
       API. It is therefore not possible in general to obtain the exact number
       of characters that must be retained in order to get the right match re-
       sult. If you cannot retain the  entire  segment,  you  must  find  some
       heuristic way of choosing.

       If  you know the approximate length of the matching substrings, you can
       use that to decide how much text to retain. The only lookbehind  infor-
       mation  that  is  currently  available via the API is the length of the
       longest individual lookbehind in a pattern, but this can be  misleading
       if  there  are  nested  lookbehinds.  The  value  returned  by  calling
       pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND  option  is  the
       maximum number of characters (not code units) that any individual look-
       behind   moves   back   when   it  is  processed.  A  pattern  such  as
       "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects  two
       characters before its starting point.

       In  a  non-UTF or a 32-bit case, moving back is just a subtraction, but
       in UTF-8 or UTF-16 you have  to  count  characters  while  moving  back
       through the code units.


PARTIAL MATCHING USING pcre2_dfa_match()

       The DFA function moves along the subject string character by character,
       without  backtracking,  searching  for  all possible matches simultane-
       ously. If the end of the subject is reached before the end of the  pat-
       tern, there is the possibility of a partial match.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there  have  been  no complete matches. Otherwise, the complete matches
       are returned.  If PCRE2_PARTIAL_HARD is  set,  a  partial  match  takes
       precedence  over  any  complete matches. The portion of the string that
       was matched when the longest partial match was  found  is  set  as  the
       first matching string.

       Because  the DFA function always searches for all possible matches, and
       there is no difference between greedy and ungreedy repetition, its  be-
       haviour  is different from the pcre2_match(). Consider the string "dog"
       matched against this ungreedy pattern:

         /dog(sbody)??/

       Whereas the standard function stops as soon as it  finds  the  complete
       match  for  "dog",  the  DFA  function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.


MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()

       When a partial match has been found using the DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. You can set the  PCRE2_PARTIAL_SOFT  or
       PCRE2_PARTIAL_HARD  options  with PCRE2_DFA_RESTART to continue partial
       matching over multiple segments. Here is an example using pcre2test:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\=dfa,ps
         Partial match: 23ja
         data> n05\=dfa,dfa_restart
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE2 does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.  This  means  that, for an unanchored pattern, if a continued match
       fails, it is not possible to try again at a  new  starting  point.  All
       this facility is capable of doing is continuing with the previous match
       attempt. For example, consider this pattern:

         1234|3789

       If  the  first  part of the subject is "ABC123", a partial match of the
       first alternative is found at offset 3. There is no partial  match  for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at one point in the subject are remembered. Depending on the ap-
       plication, this may or may not be what you want.

       If you do want to allow for starting again at the next  character,  one
       way  of  doing it is to retain some or all of the segment and try a new
       complete match, as described for pcre2_match() above. Another possibil-
       ity is to work with two buffers. If a partial match at offset n in  the
       first  buffer  is followed by "no match" when PCRE2_DFA_RESTART is used
       on the second buffer, you can then try a new match starting  at  offset
       n+1 in the first buffer.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 04 September 2019
       Copyright (c) 1997-2019 University of Cambridge.


PCRE2 10.34                    04 September 2019               PCRE2PARTIAL(3)
------------------------------------------------------------------------------



PCRE2PATTERN(3)            Library Functions Manual            PCRE2PATTERN(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference syn-
       tax summary in the pcre2syntax page. PCRE2 tries to match  Perl  syntax
       and  semantics as closely as it can.  PCRE2 also supports some alterna-
       tive regular expression syntax (which does not conflict with  the  Perl
       syntax) in order to provide some compatibility with regular expressions
       in Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of which have copious examples. Jeffrey Friedl's "Mastering Regular Ex-
       pressions",  published by O'Reilly, covers regular expressions in great
       detail. This description of PCRE2's regular expressions is intended  as
       reference material.

       This  document  discusses the regular expression patterns that are sup-
       ported by PCRE2 when its  main  matching  function,  pcre2_match(),  is
       used.    PCRE2    also    has   an   alternative   matching   function,
       pcre2_dfa_match(), which matches using a different  algorithm  that  is
       not  Perl-compatible.  Some  of  the  features  discussed below are not
       available when DFA matching is used. The advantages  and  disadvantages
       of  the  alternative function, and how it differs from the normal func-
       tion, are discussed in the pcre2matching page.


SPECIAL START-OF-PATTERN ITEMS

       A number of options that can be passed to pcre2_compile() can  also  be
       set by special items at the start of a pattern. These are not Perl-com-
       patible,  but  are provided to make these options accessible to pattern
       writers who are not able to change the program that processes the  pat-
       tern.  Any  number  of these items may appear, but they must all be to-
       gether right at the start of the pattern string, and the  letters  must
       be in upper case.

   UTF support

       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can  be  specified  for the 32-bit library, in which case it constrains
       the character values to valid  Unicode  code  points.  To  process  UTF
       strings,  PCRE2  must be built to include Unicode support (which is the
       default). When using UTF strings you must  either  call  the  compiling
       function  with  one or both of the PCRE2_UTF or PCRE2_MATCH_INVALID_UTF
       options, or the pattern must start with the  special  sequence  (*UTF),
       which  is  equivalent  to setting the relevant PCRE2_UTF. How setting a
       UTF mode affects pattern matching is mentioned in several places below.
       There is also a summary of features in the pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  to  non-UTF  data  for   security   reasons.   If   the
       PCRE2_NEVER_UTF  option is passed to pcre2_compile(), (*UTF) is not al-
       lowed, and its appearance in a pattern causes an error.

   Unicode property support

       Another special sequence that may appear at the start of a  pattern  is
       (*UCP).   This  has the same effect as setting the PCRE2_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to  deter-
       mine character types, instead of recognizing only characters with codes
       less than 256 via a lookup table. If also causes upper/lower casing op-
       erations  to  use  Unicode  properties  for characters with code points
       greater than 127, even when UTF is not set.  These  behaviours  can  be
       changed  within  the pattern; see the section entitled "Internal Option
       Setting" below.

       Some applications that allow their users to supply patterns may wish to
       restrict them for security reasons. If the  PCRE2_NEVER_UCP  option  is
       passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in
       a pattern causes an error.

   Locking out empty string matching

       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect  as  passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option
       to whichever matching function is subsequently called to match the pat-
       tern. These options lock out the matching of empty strings, either  en-
       tirely, or only at the start of the subject.

   Disabling auto-possessification

       If  a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
       setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from  making
       quantifiers  possessive  when  what  follows  cannot match the repeated
       item. For example, by default a+b is treated as a++b. For more details,
       see the pcre2api documentation.

   Disabling start-up optimizations

       If a pattern starts with (*NO_START_OPT), it has  the  same  effect  as
       setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti-
       mizations  for  quickly  reaching "no match" results. For more details,
       see the pcre2api documentation.

   Disabling automatic anchoring

       If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the  same  effect
       as  setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza-
       tions that apply to patterns whose top-level branches all start with .*
       (match any number of arbitrary characters). For more details,  see  the
       pcre2api documentation.

   Disabling JIT compilation

       If  a  pattern  that starts with (*NO_JIT) is successfully compiled, an
       attempt by the application to apply the  JIT  optimization  by  calling
       pcre2_jit_compile() is ignored.

   Setting match resource limits

       The pcre2_match() function contains a counter that is incremented every
       time it goes round its main loop. The caller of pcre2_match() can set a
       limit  on  this counter, which therefore limits the amount of computing
       resource used for a match. The maximum depth of nested backtracking can
       also be limited; this indirectly restricts the amount  of  heap  memory
       that  is  used,  but there is also an explicit memory limit that can be
       set.

       These facilities are provided to catch runaway matches  that  are  pro-
       voked  by patterns with huge matching trees. A common example is a pat-
       tern with nested unlimited repeats applied to a long string  that  does
       not  match. When one of these limits is reached, pcre2_match() gives an
       error return. The limits can also be set by items at the start  of  the
       pattern of the form

         (*LIMIT_HEAP=d)
         (*LIMIT_MATCH=d)
         (*LIMIT_DEPTH=d)

       where d is any number of decimal digits. However, the value of the set-
       ting  must  be  less than the value set (or defaulted) by the caller of
       pcre2_match() for it to have any effect. In other  words,  the  pattern
       writer  can lower the limits set by the programmer, but not raise them.
       If there is more than one setting of one of  these  limits,  the  lower
       value  is used. The heap limit is specified in kibibytes (units of 1024
       bytes).

       Prior to release 10.30, LIMIT_DEPTH was  called  LIMIT_RECURSION.  This
       name is still recognized for backwards compatibility.

       The heap limit applies only when the pcre2_match() or pcre2_dfa_match()
       interpreters are used for matching. It does not apply to JIT. The match
       limit  is used (but in a different way) when JIT is being used, or when
       pcre2_dfa_match() is called, to limit computing resource usage by those
       matching functions. The depth limit is ignored by JIT but  is  relevant
       for  DFA  matching, which uses function recursion for recursions within
       the pattern and for lookaround assertions and atomic  groups.  In  this
       case, the depth limit controls the depth of such recursion.

   Newline conventions

       PCRE2  supports six different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  any  Unicode  newline  sequence,  or the NUL character (binary
       zero). The pcre2api page has further  discussion  about  newlines,  and
       shows how to set the newline convention when calling pcre2_compile().

       It  is also possible to specify a newline convention by starting a pat-
       tern string with one of the following sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences
         (*NUL)       the NUL character (binary zero)

       These override the default and the options given to the compiling func-
       tion. For example, on a Unix system where LF is the default newline se-
       quence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The newline convention affects where the circumflex and  dollar  asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter  when  PCRE2_DOTALL  is not set, and the behaviour of \N when not
       followed by an opening brace. However, it does not affect what  the  \R
       escape  sequence  matches.  By default, this is any Unicode newline se-
       quence, for Perl compatibility. However, this can be changed;  see  the
       next section and the description of \R in the section entitled "Newline
       sequences"  below. A change of \R setting can be combined with a change
       of newline convention.

   Specifying what \R matches

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE2_BSR_ANYCRLF  at compile time. This effect can also be achieved by
       starting a pattern with (*BSR_ANYCRLF).  For  completeness,  (*BSR_UNI-
       CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


EBCDIC CHARACTER CODES

       PCRE2  can be compiled to run in an environment that uses EBCDIC as its
       character code instead of ASCII or Unicode (typically a mainframe  sys-
       tem).  In  the  sections below, character code values are ASCII or Uni-
       code; in an EBCDIC environment these characters may have different code
       values, and there are no code points greater than 255.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless  matching  is  specified  (the  PCRE2_CASELESS  option or (?i)
       within the pattern), letters are matched independently  of  case.  Note
       that  there  are  two  ASCII  characters, K and S, that, in addition to
       their lower case ASCII equivalents, are  case-equivalent  with  Unicode
       U+212A  (Kelvin  sign)  and  U+017F  (long  S) respectively when either
       PCRE2_UTF or PCRE2_UCP is set, unless the PCRE2_EXTRA_CASELESS_RESTRICT
       option is in force (either passed to pcre2_compile()  or  set  by  (?r)
       within the pattern).

       The power of regular expressions comes from the ability to include wild
       cards, character classes, alternatives, and repetitions in the pattern.
       These are encoded in the pattern by the use of metacharacters, which do
       not  stand  for  themselves but instead are interpreted in some special
       way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start group or control verb
         )      end group or control verb
         *      0 or more quantifier
         +      1 or more quantifier; also "possessive quantifier"
         ?      0 or 1 quantifier; also quantifier minimizer
         {      potential start of min/max quantifier

       Brace  characters  {  and } are also used to enclose data for construc-
       tions such as \g{2} or \k{name}. In almost all uses  of  braces,  space
       and/or horizontal tab characters that follow { or precede } are allowed
       and  are  ignored. In the case of quantifiers, they may also appear be-
       fore or after the comma. The exception to this is \u{...} which  is  an
       ECMAScript  compatibility  feature  that  is  recognized  only when the
       PCRE2_EXTRA_ALT_BSUX option is set. ECMAScript  does  not  ignore  such
       white space; it causes the item to be interpreted as literal.

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (if followed by POSIX syntax)
         ]      terminates the character class

       If a pattern is compiled with the  PCRE2_EXTENDED  option,  most  white
       space in the pattern, other than in a character class, within a \Q...\E
       sequence,  or  between  a # outside a character class and the next new-
       line, inclusive, are ignored. An escaping backslash can be used to  in-
       clude  a  white  space  or a # character as part of the pattern. If the
       PCRE2_EXTENDED_MORE option is set, the same applies,  but  in  addition
       unescaped  space  and  horizontal  tab  characters are ignored inside a
       character class. Note: only these two characters are ignored,  not  the
       full  set  of pattern white space characters that are ignored outside a
       character class. Option settings can be changed within a  pattern;  see
       the section entitled "Internal Option Setting" below.

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a  character that is not a digit or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if you want to match a * character, you must write \* in
       the pattern. This escaping action applies whether or not the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that it stands for itself.  In particular, if you want to match a back-
       slash, you write \\.

       Only  ASCII  digits  and letters have any special meaning after a back-
       slash. All other characters (in particular, those whose code points are
       greater than 127) are treated as literals.

       If you want to treat all characters in a sequence as literals, you  can
       do  so by putting them between \Q and \E. Note that this includes white
       space even when the PCRE2_EXTENDED option is set  so  that  most  other
       white  space is ignored. The behaviour is different from Perl in that $
       and @ are handled as literals in \Q...\E sequences in PCRE2, whereas in
       Perl, $ and @ cause variable interpolation. Also,  Perl  does  "double-
       quotish  backslash  interpolation" on any backslashes between \Q and \E
       which, its documentation says, "may lead to confusing  results".  PCRE2
       treats  a  backslash  between  \Q and \E just like any other character.
       Note the following examples:

         Pattern            PCRE2 matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
         \QA\B\E            A\B            A\B
         \Q\\E              \              \\E

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.   An  isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the pattern, the literal  interpretation
       continues  to  the  end  of  the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class,  this  causes  an
       error,  because the character class is then not terminated by a closing
       square bracket.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters in a pattern, but when a pattern
       is being prepared by text editing, it is often easier to use one of the
       following escape sequences instead of the binary  character  it  repre-
       sents.  In  an  ASCII or Unicode environment, these escapes are as fol-
       lows:

         \a          alarm, that is, the BEL character (hex 07)
         \cx         "control-x", where x is a non-control ASCII character
         \e          escape (hex 1B)
         \f          form feed (hex 0C)
         \n          linefeed (hex 0A)
         \r          carriage return (hex 0D) (but see below)
         \t          tab (hex 09)
         \0dd        character with octal code 0dd
         \ddd        character with octal code ddd, or backreference
         \o{ddd..}   character with octal code ddd..
         \xhh        character with hex code hh
         \x{hhh..}   character with hex code hhh..
         \N{U+hhh..} character with Unicode hex code point hhh..

       By default, after \x that is not followed by {, from zero to two  hexa-
       decimal  digits  are  read (letters can be in upper or lower case). Any
       number of hexadecimal digits may appear between \x{ and }. If a charac-
       ter other than a hexadecimal digit appears between \x{  and  },  or  if
       there is no terminating }, an error occurs.

       Characters whose code points are less than 256 can be defined by either
       of the two syntaxes for \x or by an octal sequence. There is no differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as  \x{dc}  or \334.  However, using the braced versions does make such
       sequences easier to read.

       Support is available for some ECMAScript (aka  JavaScript)  escape  se-
       quences via two compile-time options. If PCRE2_ALT_BSUX is set, the se-
       quence  \x  followed  by { is not recognized. Only if \x is followed by
       two hexadecimal digits is it recognized as a character  escape.  Other-
       wise  it  is interpreted as a literal "x" character. In this mode, sup-
       port for code points greater than 256 is provided by \u, which must  be
       followed  by  four hexadecimal digits; otherwise it is interpreted as a
       literal "u" character.

       PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in  ad-
       dition, \u{hhh..} is recognized as the character specified by hexadeci-
       mal code point.  There may be any number of hexadecimal digits, but un-
       like  other places that also use curly brackets, spaces are not allowed
       and would result in the string being interpreted  as  a  literal.  This
       syntax is from ECMAScript 6.

       The  \N{U+hhh..} escape sequence is recognized only when PCRE2 is oper-
       ating in UTF mode. Perl also uses \N{name}  to  specify  characters  by
       Unicode  name;  PCRE2  does  not support this. Note that when \N is not
       followed by an opening brace (curly bracket) it has an entirely differ-
       ent meaning, matching any character that is not a newline.

       There are some legacy applications where the escape sequence \r is  ex-
       pected  to  match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option
       is set, \r in a pattern is converted to \n so  that  it  matches  a  LF
       (linefeed) instead of a CR (carriage return) character.

       An  error  occurs if \c is not followed by a character whose ASCII code
       point is in the range 32 to 126. The precise effect of \cx is  as  fol-
       lows:  if x is a lower case letter, it is converted to upper case. Then
       bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex
       01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B),  and
       \c;  becomes hex 7B (; is 3B). If the code unit following \c has a code
       point less than 32 or greater than 126, a compile-time error occurs.

       When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..}  is  not  supported.
       \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values.
       The \c escape is processed as specified for Perl in the perlebcdic doc-
       ument.  The  only characters that are allowed after \c are A-Z, a-z, or
       one of @, [, \, ], ^, _, or ?. Any other character provokes a  compile-
       time  error.  The  sequence  \c@ encodes character code 0; after \c the
       letters (in either case) encode characters 1-26 (hex 01 to hex 1A);  [,
       \,  ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? be-
       comes either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the  same  character  code
       values  as  they do in an ASCII environment, though the meanings of the
       values mostly differ. For example, \cG always generates code  value  7,
       which is BEL in ASCII but DEL in EBCDIC.

       The  sequence  \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in  EBCDIC,  Perl  makes  it
       generate  the  APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has  the  value  255  (hex
       FF),  but  in  the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE2 makes \c? generate
       95; otherwise it generates 255.

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those that are present are used. Thus the se-
       quence \0\x\015 specifies two binary zeros followed by a  CR  character
       (code value 13). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The  escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case. This  escape  is  a
       recent  addition  to Perl; it provides way of specifying character code
       points as octal numbers greater than 0777, and  it  also  allows  octal
       numbers and backreferences to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a  digit  greater than zero. Instead, use \o{...} or \x{...} to specify
       numerical character code points, and \g{...} to specify backreferences.
       The following paragraphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated, and Perl has changed over time, causing PCRE2 also to change.

       Outside a character class, PCRE2 reads the digit and any following dig-
       its as a decimal number. If the number is less than 10, begins with the
       digit 8 or 9, or if there are  at  least  that  many  previous  capture
       groups  in the expression, the entire sequence is taken as a backrefer-
       ence. A description of how this works is  given  later,  following  the
       discussion  of parenthesized groups.  Otherwise, up to three octal dig-
       its are read to form a character code.

       Inside a character class, PCRE2 handles \8 and \9 as the literal  char-
       acters  "8"  and "9", and otherwise reads up to three octal digits fol-
       lowing the backslash, using them to generate a data character. Any sub-
       sequent digits stand for themselves. For example, outside  a  character
       class:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capture groups
         \7     is always a backreference
         \11    might be a backreference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a backreference, otherwise the
                   character with octal code 113
         \377   might be a backreference, otherwise
                   the value 255 (decimal)
         \81    is always a backreference

       Note  that octal values of 100 or greater that are specified using this
       syntax must not be introduced by a leading zero, because no  more  than
       three octal digits are ever read.

   Constraints on character values

       Characters  that  are  specified using octal or hexadecimal numbers are
       limited to certain values, as follows:

         8-bit non-UTF mode    no greater than 0xff
         16-bit non-UTF mode   no greater than 0xffff
         32-bit non-UTF mode   no greater than 0xffffffff
         All UTF modes         no greater than 0x10ffff and a valid code point

       Invalid Unicode code points are all those in the range 0xd800 to 0xdfff
       (the so-called "surrogate" code points). The check  for  these  can  be
       disabled  by  the  caller  of  pcre2_compile()  by  setting  the option
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only  in
       UTF-8  and  UTF-32 modes, because these values are not representable in
       UTF-16.

   Escape sequences in character classes

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

       When not followed by an opening brace, \N is not allowed in a character
       class.   \B,  \R, and \X are not special inside a character class. Like
       other unrecognized alphabetic escape sequences, they  cause  an  error.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences

       In  Perl,  the  sequences  \F, \l, \L, \u, and \U are recognized by its
       string handler and used to modify the case of following characters.  By
       default,  PCRE2  does  not  support these escape sequences in patterns.
       However, if either of the PCRE2_ALT_BSUX  or  PCRE2_EXTRA_ALT_BSUX  op-
       tions  is set, \U matches a "U" character, and \u can be used to define
       a character by code point, as described above.

   Absolute and relative backreferences

       The sequence \g followed by a signed or unsigned number, optionally en-
       closed in braces, is an absolute or  relative  backreference.  A  named
       backreference  can  be  coded as \g{name}. Backreferences are discussed
       later, following the discussion of parenthesized groups.

   Absolute and relative subroutine calls

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative syntax for referencing a capture group as a subroutine.
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a backref-
       erence; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \N     any character that is not a newline
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       The  \N  escape  sequence has the same meaning as the "." metacharacter
       when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not  change
       the meaning of \N. Note that when \N is followed by an opening brace it
       has a different meaning. See the section entitled "Non-printing charac-
       ters"  above for details. Perl also uses \N{name} to specify characters
       by Unicode name; PCRE2 does not support this.

       Each pair of lower and upper case escape sequences partitions the  com-
       plete  set  of  characters  into two disjoint sets. Any given character
       matches one, and only one, of each pair. The sequences can appear  both
       inside  and outside character classes. They each match one character of
       the appropriate type. If the current matching point is at  the  end  of
       the  subject string, all of them fail, because there is no character to
       match.

       The default \s characters are HT (9), LF (10), VT  (11),  FF  (12),  CR
       (13),  and  space (32), which are defined as white space in the "C" lo-
       cale. This list may vary if locale-specific matching is  taking  place.
       For  example, in some locales the "non-breaking space" character (\xA0)
       is recognized as white space, and in others the VT character is not.

       A "word" character is an underscore or any character that is  a  letter
       or  digit.   By  default,  the definition of letters and digits is con-
       trolled by PCRE2's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the pcre2api
       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 127
       are used for accented letters, and these are then matched  by  \w.  The
       use of locales with Unicode is discouraged.

       By  default,  characters  whose  code points are greater than 127 never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be different for characters in the range 128-255  when  locale-specific
       matching  is  happening.   These escape sequences retain their original
       meanings from before Unicode support was available,  mainly  for  effi-
       ciency  reasons.  If  the  PCRE2_UCP  option  is  set, the behaviour is
       changed so that Unicode properties  are  used  to  determine  character
       types, as follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L}, \p{N}, \p{Mn}, or \p{Pc}

       The addition of \p{Mn} (non-spacing mark) and the replacement of an ex-
       plicit  test  for underscore with a test for \p{Pc} (connector punctua-
       tion) happened in PCRE2 release 10.43. This brings PCRE2 into line with
       Perl.

       The upper case escapes match the inverse sets of characters. Note  that
       \d  matches  only decimal digits, whereas \w matches any Unicode digit,
       as well as other character categories. Note also that PCRE2_UCP affects
       \b, and \B because they are defined in terms of  \w  and  \W.  Matching
       these sequences is noticeably slower when PCRE2_UCP is set.

       The  effect  of  PCRE2_UCP  on any one of these escape sequences can be
       negated by the  options  PCRE2_EXTRA_ASCII_BSD,  PCRE2_EXTRA_ASCII_BSS,
       and  PCRE2_EXTRA_ASCII_BSW,  respectively. These options can be set and
       reset within a pattern by means of an internal option setting (see  be-
       low).

       The  sequences  \h, \H, \v, and \V, in contrast to the other sequences,
       which match only ASCII characters by default, always match  a  specific
       list  of  code  points, whether or not PCRE2_UCP is set. The horizontal
       space characters are:

         U+0009     Horizontal tab (HT)
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters  with  code  points  less
       than 256 are relevant.

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is  equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given be-
       low.   This  particular group matches either the two-character sequence
       CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
       U+000A),  VT  (vertical  tab, U+000B), FF (form feed, U+000C), CR (car-
       riage return, U+000D), or NEL (next line, U+0085). Because this  is  an
       atomic  group,  the  two-character sequence is treated as a single unit
       that cannot be split.

       In other modes, two additional characters whose code points are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode support is not needed for these characters  to
       be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the  complete  set  of  Unicode  line  endings)  by  setting the option
       PCRE2_BSR_ANYCRLF at compile time. (BSR is an abbreviation  for  "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the  case,  the other behaviour can be requested via the PCRE2_BSR_UNI-
       CODE option. It is also possible to specify these settings by  starting
       a pattern string with one of the following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are  recognized only at the very start of a pattern, and that they must
       be in upper case. If more than one of them is present, the last one  is
       used. They can be combined with a change of newline convention; for ex-
       ample, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They  can also be combined with the (*UTF) or (*UCP) special sequences.
       Inside a character class, \R is treated as an unrecognized  escape  se-
       quence, and causes an error.

   Unicode character properties

       When  PCRE2  is  built  with Unicode support (the default), three addi-
       tional escape sequences that match characters with specific  properties
       are available. They can be used in any mode, though in 8-bit and 16-bit
       non-UTF  modes these sequences are of course limited to testing charac-
       ters whose code points are less than U+0100 and U+10000,  respectively.
       In  32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode
       limit) may be encountered. These are all treated as being  in  the  Un-
       known script and with an unassigned type.

       Matching  characters by Unicode property is not fast, because PCRE2 has
       to do a multistage table lookup in order to find  a  character's  prop-
       erty. That is why the traditional escape sequences such as \d and \w do
       not  use  Unicode  properties  in PCRE2 by default, though you can make
       them do so by setting the PCRE2_UCP option or by starting  the  pattern
       with (*UCP).

       The extra escape sequences that provide property support are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       a Unicode extended grapheme cluster

       The  property names represented by xx above are not case-sensitive, and
       in accordance with Unicode's "loose matching" rules,  spaces,  hyphens,
       and underscores are ignored. There is support for Unicode script names,
       Unicode general category properties, "Any", which matches any character
       (including  newline),  Bidi_Class,  a number of binary (yes/no) proper-
       ties, and some special PCRE2  properties  (described  below).   Certain
       other  Perl  properties such as "InMusicalSymbols" are not supported by
       PCRE2. Note that \P{Any} does  not  match  any  characters,  so  always
       causes a match failure.

   Script properties for \p and \P

       There are three different syntax forms for matching a script. Each Uni-
       code  character  has  a  basic  script and, optionally, a list of other
       scripts ("Script Extensions") with which it is commonly used. Using the
       Adlam script as an example, \p{sc:Adlam} matches characters whose basic
       script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters
       that have Adlam in their extensions list. The full names  "script"  and
       "script extensions" for the property types are recognized, and a equals
       sign  is an alternative to the colon. If a script name is given without
       a property type, for example, \p{Adlam}, it is  treated  as  \p{scx:Ad-
       lam}.  Perl  changed  to  this interpretation at release 5.26 and PCRE2
       changed at release 10.40.

       Unassigned characters (and in non-UTF 32-bit mode, characters with code
       points greater than 0x10FFFF) are assigned the "Unknown" script. Others
       that are not part of an identified script are lumped together as  "Com-
       mon". The current list of recognized script names and their 4-character
       abbreviations can be obtained by running this command:

         pcre2test -LS


   The general category property for \p and \P

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral category properties that start with that letter. In this case,  in
       the  absence of negation, the curly brackets in the escape sequence are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The special property LC, which has the synonym L&, is  also  supported:
       it  matches  a  character that has the Lu, Ll, or Lt property, in other
       words, a letter that is not classified as a modifier or "other".

       The Cs (Surrogate) property  applies  only  to  characters  whose  code
       points  are in the range U+D800 to U+DFFF. These characters are no dif-
       ferent to any other character when PCRE2 is not in UTF mode (using  the
       16-bit  or  32-bit  library).   However,  they are not valid in Unicode
       strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid-
       ity  checking  has   been   turned   off   (see   the   discussion   of
       PCRE2_NO_UTF_CHECK in the pcre2api page).

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper  case  letters.  This  is
       different from the behaviour of current versions of Perl.

   Binary (yes/no) properties for \p and \P

       Unicode  defines  a  number  of  binary properties, that is, properties
       whose only values are true or false. You can obtain  a  list  of  those
       that  are  recognized  by \p and \P, along with their abbreviations, by
       running this command:

         pcre2test -LP


   The Bidi_Class property for \p and \P

         \p{Bidi_Class:<class>}   matches a character with the given class
         \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

         AL          Arabic letter
         AN          Arabic number
         B           paragraph separator
         BN          boundary neutral
         CS          common separator
         EN          European number
         ES          European separator
         ET          European terminator
         FSI         first strong isolate
         L           left-to-right
         LRE         left-to-right embedding
         LRI         left-to-right isolate
         LRO         left-to-right override
         NSM         non-spacing mark
         ON          other neutral
         PDF         pop directional format
         PDI         pop directional isolate
         R           right-to-left
         RLE         right-to-left embedding
         RLI         right-to-left isolate
         RLO         right-to-left override
         S           segment separator
         WS          which space

       An equals sign may be used instead of a  colon.  The  class  names  are
       case-insensitive; only the short names listed above are recognized.

   Extended grapheme clusters

       The  \X  escape  matches  any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Unicode supports various kinds of composite character  by
       giving  each  character  a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters. The rules are defined in Unicode Standard Annex 29,  "Unicode
       Text  Segmentation".  Unicode 11.0.0 abandoned the use of some previous
       properties that had been used for emojis.  Instead it introduced  vari-
       ous  emoji-specific  properties.  PCRE2  uses  only the Extended Picto-
       graphic property.

       \X always matches at least one character. Then it  decides  whether  to
       add additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the subject string.

       2.  Do not end between CR and LF; otherwise end after any control char-
       acter.

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be  followed  by  a V or T character; an LVT or T character may be fol-
       lowed only by a T character.

       4. Do not end before extending characters or spacing marks or the zero-
       width joiner (ZWJ) character. Characters with the "mark"  property  al-
       ways have the "extend" grapheme breaking property.

       5. Do not end after prepend characters.

       6.  Do not end within emoji modifier sequences or emoji ZWJ (zero-width
       joiner) sequences. An emoji ZWJ sequence consists of a  character  with
       the  Extended_Pictographic property, optionally followed by one or more
       characters with the Extend property, followed  by  the  ZWJ  character,
       followed by another Extended_Pictographic character.

       7.  Do not break within emoji flag sequences. That is, do not break be-
       tween regional indicator (RI) characters if there are an odd number  of
       RI characters before the break point.

       8. Otherwise, end the cluster.

   PCRE2's additional properties

       As  well as the standard Unicode properties described above, PCRE2 sup-
       ports four more that make it possible to convert traditional escape se-
       quences such as \w and \s to use Unicode properties. PCRE2  uses  these
       non-standard,  non-Perl  properties  internally  when PCRE2_UCP is set.
       However, they may also be used explicitly. These properties are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the  N  (num-
       ber)  property. Xps matches the characters tab, linefeed, vertical tab,
       form feed, or carriage return, and any other character that has  the  Z
       (separator)  property.  Xsp is the same as Xps; in PCRE1 it used to ex-
       clude vertical tab, for  Perl  compatibility,  but  Perl  changed.  Xwd
       matches the same characters as Xan, plus those that match Mn (non-spac-
       ing mark) or Pc (connector punctuation, which includes underscore).

       There  is another non-standard property, Xuc, which matches any charac-
       ter that can be represented by a Universal Character Name  in  C++  and
       other  programming  languages.  These are the characters $, @, ` (grave
       accent), and all characters with Unicode code points  greater  than  or
       equal  to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that
       most base (ASCII) characters are excluded. (Universal  Character  Names
       are  of  the  form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they represent.)

   Resetting the match start

       In normal use, the escape sequence \K  causes  any  previously  matched
       characters not to be included in the final matched sequence that is re-
       turned. For example, the pattern:

         foo\Kbar

       matches  "foobar",  but  reports that it has matched "bar". \K does not
       interact with anchoring in any way. The pattern:

         ^foo\Kbar

       matches only when the subject begins  with  "foobar"  (in  single  line
       mode),  though  it again reports the matched string as "bar". This fea-
       ture is similar to a lookbehind assertion (described  below),  but  the
       part of the pattern that precedes \K is not constrained to match a lim-
       ited  number  of characters, as is required for a lookbehind assertion.
       The use of \K does not interfere with  the  setting  of  captured  sub-
       strings.  For example, when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       From  version  5.32.0  Perl  forbids the use of \K in lookaround asser-
       tions. From release 10.38 PCRE2 also forbids this by default.  However,
       the  PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK  option  can be used when calling
       pcre2_compile() to re-enable the previous behaviour. When  this  option
       is set, \K is acted upon when it occurs inside positive assertions, but
       is  ignored  in  negative  assertions. Note that when a pattern such as
       (?=ab\K) matches, the reported start of the match can be  greater  than
       the  end  of the match. Using \K in a lookbehind assertion at the start
       of a pattern can also lead to odd effects. For example,  consider  this
       pattern:

         (?<=\Kfoo)bar

       If  the  subject  is  "foobar", a call to pcre2_match() with a starting
       offset of 3 succeeds and reports the matching string as "foobar",  that
       is,  the  start  of  the reported match is earlier than where the match
       started.

   Simple assertions

       The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of groups for more complicated assertions is described below.  The
       backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning;  it  matches  the
       backspace  character.  If  any  other  of these assertions appears in a
       character class, an "invalid escape sequence" error is generated.

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string  if  the  first or last character matches \w, respectively. When
       PCRE2 is built with Unicode support, the meanings of \w and \W  can  be
       changed by setting the PCRE2_UCP option. When this is done, it also af-
       fects  \b and \B. Neither PCRE2 nor Perl has a separate "start of word"
       or "end of word" metasequence. However, whatever  follows  \b  normally
       determines  which  it  is. For example, the fragment \ba matches "a" at
       the start of a word.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions  are  not  affected  by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
       which affect only the behaviour of the circumflex and dollar  metachar-
       acters.  However,  if the startoffset argument of pcre2_match() is non-
       zero, indicating that matching is to start at a point  other  than  the
       beginning  of  the subject, \A can never match.  The difference between
       \Z and \z is that \Z matches before a newline at the end of the  string
       as well as at the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the matching process, as specified by the  startoff-
       set  argument  of  pcre2_match().  It differs from \A when the value of
       startoffset is non-zero. By calling pcre2_match() multiple  times  with
       appropriate  arguments,  you  can  mimic Perl's /g option, and it is in
       this kind of implementation where \G can be useful.

       Note, however, that PCRE2's implementation of \G,  being  true  at  the
       starting  character  of  the matching process, is subtly different from
       Perl's, which defines it as true at the end of the previous  match.  In
       Perl,  these  can  be  different when the previously matched string was
       empty. Because PCRE2 does just one match at a time, it cannot reproduce
       this behaviour.

       If all the alternatives of a pattern begin with \G, the  expression  is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       The  circumflex  and  dollar  metacharacters are zero-width assertions.
       That is, they test for a particular condition being true  without  con-
       suming any characters from the subject string. These two metacharacters
       are  concerned  with matching the starts and ends of lines. If the new-
       line convention is set so that only the two-character sequence CRLF  is
       recognized  as  a newline, isolated CR and LF characters are treated as
       ordinary data characters, and are not recognized as newlines.

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is  set,  circum-
       flex  can  never match if the PCRE2_MULTILINE option is unset. Inside a
       character class, circumflex has an entirely different meaning (see  be-
       low).

       Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       The  dollar  character is an assertion that is true only if the current
       matching point is at the end of the subject string, or immediately  be-
       fore  a newline at the end of the string (by default), unless PCRE2_NO-
       TEOL is set. Note, however, that it does not actually  match  the  new-
       line.  Dollar need not be the last character of the pattern if a number
       of alternatives are involved, but it should be the  last  item  in  any
       branch  in which it appears. Dollar has no special meaning in a charac-
       ter class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of the string, by setting the PCRE2_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the PCRE2_MULTILINE option is set. When this  is  the  case,  a  dollar
       character  matches before any newlines in the string, as well as at the
       very end, and a circumflex matches immediately after internal  newlines
       as  well as at the start of the subject string. It does not match after
       a newline that ends the string, for compatibility with  Perl.  However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For  example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but  not  otherwise.
       Consequently,  patterns  that  are anchored in single line mode because
       all branches start with ^ are not anchored in  multiline  mode,  and  a
       match  for  circumflex  is  possible  when  the startoffset argument of
       pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option  is  ignored
       if PCRE2_MULTILINE is set.

       When  the  newline  convention (see "Newline conventions" below) recog-
       nizes the two-character sequence CRLF as a newline, this is  preferred,
       even  if  the  single  characters CR and LF are also recognized as new-
       lines. For example, if the newline convention  is  "any",  a  multiline
       mode  circumflex matches before "xyz" in the string "abc\r\nxyz" rather
       than after CR, even though CR on its own is a valid newline.  (It  also
       matches at the very start of the string, of course.)

       Note  that  the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a  pattern
       start  with \A it is always anchored, whether or not PCRE2_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies the end of a line. One or more characters may be specified as line
       terminators (see "Newline conventions" above).

       Dot  never matches a single line-ending character. When the two-charac-
       ter sequence CRLF is the only line ending, dot does not match CR if  it
       is  immediately followed by LF, but otherwise it matches all characters
       (including isolated CRs and LFs). When ANYCRLF  is  selected  for  line
       endings,  no  occurrences  of CR of LF match dot. When all Unicode line
       endings are being recognized, dot does not match CR or LF or any of the
       other line ending characters.

       The behaviour of dot with regard to newlines can  be  changed.  If  the
       PCRE2_DOTALL  option  is  set, a dot matches any one character, without
       exception.  If the two-character sequence CRLF is present in  the  sub-
       ject string, it takes two dots to match it.

       The  handling of dot is entirely independent of the handling of circum-
       flex and dollar, the only relationship being  that  they  both  involve
       newlines. Dot has no special meaning in a character class.

       The  escape  sequence  \N when not followed by an opening brace behaves
       like a dot, except that it is not affected by the PCRE2_DOTALL  option.
       In  other words, it matches any character except one that signifies the
       end of a line.

       When \N is followed by an opening brace it has a different meaning. See
       the section entitled "Non-printing characters" above for details.  Perl
       also  uses  \N{name}  to specify characters by Unicode name; PCRE2 does
       not support this.


MATCHING A SINGLE CODE UNIT

       Outside a character class, the escape sequence \C matches any one  code
       unit,  whether or not a UTF mode is set. In the 8-bit library, one code
       unit is one byte; in the 16-bit library it is a  16-bit  unit;  in  the
       32-bit  library  it  is  a 32-bit unit. Unlike a dot, \C always matches
       line-ending characters. The feature is provided in  Perl  in  order  to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used.

       Because  \C  breaks  up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that  the  rest  of  the
       string may start with a malformed UTF character. This has undefined re-
       sults, because PCRE2 assumes that it is matching character by character
       in a valid UTF string (by default it checks the subject string's valid-
       ity  at  the  start  of  processing  unless  the  PCRE2_NO_UTF_CHECK or
       PCRE2_MATCH_INVALID_UTF option is used).

       An  application  can  lock  out  the  use  of   \C   by   setting   the
       PCRE2_NEVER_BACKSLASH_C  option  when  compiling  a pattern. It is also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2 does not allow \C to appear in lookbehind  assertions  (described
       below)  in UTF-8 or UTF-16 modes, because this would make it impossible
       to calculate the length of  the  lookbehind.  Neither  the  alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes.  The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In  the  32-bit  library, however, \C is always supported (when not ex-
       plicitly locked out) because it always  matches  a  single  code  unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it  that avoids the problem of malformed UTF-8 or UTF-16 charac-
       ters is to use a lookahead to check the length of the  next  character,
       as  in  this  pattern,  which could be used with a UTF-8 string (ignore
       white space and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       In this example, a group that starts  with  (?|  resets  the  capturing
       parentheses  numbers in each alternative (see "Duplicate Group Numbers"
       below). The assertions at the start of each branch check the next UTF-8
       character for values whose encoding uses 1, 2, 3, or 4  bytes,  respec-
       tively.  The  character's individual bytes are then captured by the ap-
       propriate number of \C groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  If a closing square bracket is required as  a  member
       of the class, it should be the first data character in the class (after
       an  initial  circumflex,  if present) or escaped with a backslash. This
       means that, by default, an empty class cannot be defined.  However,  if
       the  PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A  matched
       character must be in the set of characters defined by the class, unless
       the  first  character in the class definition is a circumflex, in which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

       For  example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a  lower  case  vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion; it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       Characters in a class may be specified by their code points  using  \o,
       \x,  or \N{U+hh..} in the usual way. When caseless matching is set, any
       letters in a class represent both their upper case and lower case  ver-
       sions,  so  for example, a caseless [aeiou] matches "A" as well as "a",
       and a caseless [^aeiou] does not match "A", whereas a  caseful  version
       would.  Note that there are two ASCII characters, K and S, that, in ad-
       dition to their lower case ASCII equivalents, are case-equivalent  with
       Unicode  U+212A (Kelvin sign) and U+017F (long S) respectively when ei-
       ther PCRE2_UTF or PCRE2_UCP is set.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when matching character classes, whatever line-ending se-
       quence is  in  use,  and  whatever  setting  of  the  PCRE2_DOTALL  and
       PCRE2_MULTILINE  options  is  used. A class such as [^a] always matches
       one of these characters.

       The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
       \S, \v, \V, \w, and \W may appear in a character  class,  and  add  the
       characters  that  they  match  to  the  class.  For example, [\dABCDEF]
       matches any hexadecimal digit. In UTF modes, the PCRE2_UCP  option  af-
       fects the meanings of \d, \s, \w and their upper case partners, just as
       it does when they appear outside a character class, as described in the
       section  entitled  "Generic character types" above. The escape sequence
       \b has a different meaning inside a character  class;  it  matches  the
       backspace  character.  The sequences \B, \R, and \X are not special in-
       side a character class. Like any other unrecognized  escape  sequences,
       they  cause  an  error. The same is true for \N when not followed by an
       opening brace.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character class. For example, [d-m] matches any letter be-
       tween d and m, inclusive. If a minus character is required in a  class,
       it  must  be  escaped with a backslash or appear in a position where it
       cannot be interpreted as indicating a range, typically as the first  or
       last character in the class, or immediately after a range. For example,
       [b-d-z] matches letters in the range b to d, a hyphen character, or z.

       Perl treats a hyphen as a literal if it appears before or after a POSIX
       class (see below) or before or after a character type escape such as \d
       or  \H.  However, unless the hyphen is the last character in the class,
       Perl outputs a warning in its warning mode, as this is  most  likely  a
       user  error. As PCRE2 has no facility for warning, an error is given in
       these cases.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges normally include all code points between the start and end char-
       acters, inclusive. They can also be used for code points specified  nu-
       merically,  for  example [\000-\037]. Ranges can include any characters
       that are valid for the current mode. In any  UTF  mode,  the  so-called
       "surrogate"  characters (those whose code points lie between 0xd800 and
       0xdfff inclusive) may not  be  specified  explicitly  by  default  (the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES  option  disables this check). How-
       ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates,
       are always permitted.

       There is a special case in EBCDIC environments  for  ranges  whose  end
       points are both specified as literal letters in the same case. For com-
       patibility  with Perl, EBCDIC code points within the range that are not
       letters are omitted. For example, [h-k] matches only  four  characters,
       even though the codes for h and k are 0x88 and 0x92, a range of 11 code
       points.  However,  if  the range is specified numerically, for example,
       [\x88-\x92] or [h-\x92], all code points are included.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name, or for  a
       special  compatibility  feature  -  see the next two sections), and the
       terminating closing square bracket.  However,  escaping  other  non-al-
       phanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by [: and :] within the enclosing square brackets. PCRE2 also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE2 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11),  FF  (12),
       CR  (13),  and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may  be  fewer  or
       more  of  them.  "Space" and \s match the same set of characters, as do
       "word" and \w.

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for charac-
       ters in the range 128-255 when locale-specific matching  is  happening.
       However,  in UCP mode, unless certain options are set (see below), some
       of the classes are changed so that  Unicode  character  properties  are
       used. This is achieved by replacing POSIX classes with other sequences,
       as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:cntrl:]  becomes  \p{Cc}
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated  versions,  such as [:^alpha:] use \P instead of \p. Four other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This matches characters that have glyphs that mark  the  page
                 when printed. In Unicode property terms, it matches all char-
                 acters with the L, M, N, P, S, or Cf properties, except for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s


       [:print:] This  matches  the  same  characters  as [:graph:] plus space
                 characters that are not controls, that  is,  characters  with
                 the Zs property.

       [:punct:] This matches all characters that have the Unicode P (punctua-
                 tion)  property,  plus those characters with code points less
                 than 256 that have the S (Symbol) property.

       [:xdigit:]
                 In addition  to  the  ASCII  hexadecimal  digits,  this  also
                 matches  the  "fullwidth" versions of those characters, whose
                 Unicode code points start at U+FF10. This is  a  change  that
                 was made in PCRE release 10.43 for Perl compatibility.

       The  other  POSIX  classes  are  unchanged by PCRE2_UCP, and match only
       characters with code points less than 256.

       There are two options that can be used to restrict the POSIX classes to
       ASCII  characters  when  PCRE2_UCP  is  set.   The   option   PCRE2_EX-
       TRA_ASCII_DIGIT  affects  just  [:digit:] and [:xdigit:]. Within a pat-
       tern, this can be set and unset by  (?aT)  and  (?-aT).  The  PCRE2_EX-
       TRA_ASCII_POSIX  option  disables UCP processing for all POSIX classes,
       including [:digit:] and [:xdigit:]. Within a pattern, (?aP) and  (?-aP)
       set and unset both these options for consistency.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES

       In  the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching  "start  of  word"
       and "end of word". PCRE2 treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b]  provokes  error  for  an unrecognized POSIX class name. This
       support is not compatible with Perl. It is provided to help  migrations
       from other environments, and is best not used in any new patterns. Note
       that  \b matches at the start and the end of a word (see "Simple asser-
       tions" above), and in a Perl-style pattern the preceding  or  following
       character  normally shows which is wanted, without the need for the as-
       sertions that are used above in order to give exactly the POSIX  behav-
       iour.  Note  also  that  the PCRE2_UCP option changes the meaning of \w
       (and therefore \b) by default, so  it  also  affects  these  POSIX  se-
       quences.


VERTICAL BAR

       Vertical  bar characters are used to separate alternative patterns. For
       example, the pattern

         gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives  may
       appear,  and  an  empty  alternative  is  permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a group (defined below), "succeeds" means matching the rest
       of the main pattern as well as the alternative in the group.


INTERNAL OPTION SETTING

       The settings of several options can be changed within a  pattern  by  a
       sequence  of  letters  enclosed between "(?" and ")". The following are
       Perl-compatible, and are described in detail in the pcre2api documenta-
       tion. The option letters are:

         i  for PCRE2_CASELESS
         m  for PCRE2_MULTILINE
         n  for PCRE2_NO_AUTO_CAPTURE
         s  for PCRE2_DOTALL
         x  for PCRE2_EXTENDED
         xx for PCRE2_EXTENDED_MORE

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the relevant letters with a hy-
       phen, for example (?-im). The two "extended" options are  not  indepen-
       dent; unsetting either one cancels the effects of both of them.

       A   combined  setting  and  unsetting  such  as  (?im-sx),  which  sets
       PCRE2_CASELESS and PCRE2_MULTILINE  while  unsetting  PCRE2_DOTALL  and
       PCRE2_EXTENDED,  is  also  permitted. Only one hyphen may appear in the
       options string. If a letter appears both before and after  the  hyphen,
       the  option  is unset. An empty options setting "(?)" is allowed. Need-
       less to say, it has no effect.

       If the first character following (? is a circumflex, it causes  all  of
       the  above  options  to  be unset. Letters may follow the circumflex to
       cause some options to be re-instated, but a hyphen may not appear.

       Some PCRE2-specific options can be changed by the same mechanism  using
       these pairs or individual letters:

         aD for PCRE2_EXTRA_ASCII_BSD
         aS for PCRE2_EXTRA_ASCII_BSS
         aW for PCRE2_EXTRA_ASCII_BSW
         aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT
         aT for PCRE2_EXTRA_ASCII_DIGIT
         r  for PCRE2_EXTRA_CASELESS_RESTRICT
         J  for PCRE2_DUPNAMES
         U  for PCRE2_UNGREEDY

       However,  except for 'r', these are not unset by (?^), which is equiva-
       lent to (?-imnrsx). If 'a' is not followed by any  of  the  upper  case
       letters shown above, it sets (or unsets) all the ASCII options.

       PCRE2_EXTRA_ASCII_DIGIT   has   no  additional  effect  when  PCRE2_EX-
       TRA_ASCII_POSIX is set, but including it in  (?aP)  means  that  (?-aP)
       suppresses all ASCII restrictions for POSIX classes.

       When  one of these option changes occurs at top level (that is, not in-
       side group parentheses), the change applies until a subsequent  change,
       or  the  end of the pattern. An option change within a group (see below
       for a description of groups) affects only that part of the  group  that
       follows  it.  At  the  end  of the group these options are reset to the
       state they were before the group. For example,

         (a(?i)b)c

       matches abc and aBc and no other strings  (assuming  PCRE2_CASELESS  is
       not  set  externally).  Any changes made in one alternative do carry on
       into subsequent branches within the same group. For example,

         (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There  would  be
       some very weird behaviour otherwise.

       As  a  convenient shorthand, if any option settings are required at the
       start of a non-capturing group (see the next section), the option  let-
       ters may appear between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings.

       Note:  There  are  other  PCRE2-specific options, applying to the whole
       pattern, which can be set by the application when the  compiling  func-
       tion  is  called.  In addition, the pattern can contain special leading
       sequences such as (*CRLF) to override what the application has  set  or
       what  has  been  defaulted.   Details are given in the section entitled
       "Newline sequences" above. There are also the (*UTF) and (*UCP) leading
       sequences that can be used to set UTF and Unicode property modes;  they
       are  equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respec-
       tively.  However,  the  application  can  set  the  PCRE2_NEVER_UTF  or
       PCRE2_NEVER_UCP  options,  which  lock  out  the  use of the (*UTF) and
       (*UCP) sequences.


GROUPS

       Groups are delimited by parentheses  (round  brackets),  which  can  be
       nested.  Turning part of a pattern into a group does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches  "cataract",  "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2. It creates a "capture group". This means that, when the  whole  pat-
       tern  matches, the portion of the subject string that matched the group
       is passed back to the caller, separately from the portion that  matched
       the  whole  pattern.   (This  applies  only to the traditional matching
       function; the DFA matching function does not support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain numbers for capture groups. For example, if the string "the  red
       king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when grouping is required without  cap-
       turing.  If an opening parenthesis is followed by a question mark and a
       colon, the group does not do any capturing, and  is  not  counted  when
       computing  the number of any subsequent capture groups. For example, if
       the string "the white queen" is matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capture groups is 65535.

       As a convenient shorthand, if any option settings are required  at  the
       start  of  a non-capturing group, the option letters may appear between
       the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until  the  end  of
       the  group is reached, an option setting in one branch does affect sub-
       sequent branches, so the above patterns match "SUNDAY" as well as "Sat-
       urday".


DUPLICATE GROUP NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a group uses
       the same numbers for its capturing parentheses.  Such  a  group  starts
       with  (?|  and  is  itself a non-capturing group. For example, consider
       this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  parentheses  are  numbered one. Thus, when the pattern matches,
       you can look at captured substring number  one,  whichever  alternative
       matched.  This  construct  is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses are numbered as usual, but the number is reset at the  start  of
       each  branch.  The numbers of any capturing parentheses that follow the
       whole group start after the highest number used in any branch. The fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A backreference to a capture group uses the most recent value  that  is
       set for the group. The following pattern matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In  contrast, a subroutine call to a capture group always refers to the
       first one in the pattern with the given number. The  following  pattern
       matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       A relative reference such as (?-1) is no different: it is just a conve-
       nient way of computing an absolute group number.

       If a condition test for a group's having matched refers to a non-unique
       number, the test is true if any group with that number has matched.

       An  alternative approach to using this "branch reset" feature is to use
       duplicate named groups, as described in the next section.


NAMED CAPTURE GROUPS

       Identifying capture groups by number is simple, but it can be very hard
       to keep track of the numbers in complicated patterns.  Furthermore,  if
       an  expression  is  modified, the numbers may change. To help with this
       difficulty, PCRE2 supports the naming of capture groups.  This  feature
       was  not  added to Perl until release 5.10. Python had the feature ear-
       lier, and PCRE1 introduced it at release 4.0, using the Python  syntax.
       PCRE2 supports both the Perl and the Python syntax.

       In  PCRE2,  a  capture  group  can  be  named  in  one  of  three ways:
       (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
       Names may be up to 128 code units long. When PCRE2_UTF is not set, they
       may contain only ASCII alphanumeric  characters  and  underscores,  but
       must start with a non-digit. When PCRE2_UTF is set, the syntax of group
       names is extended to allow any Unicode letter or Unicode decimal digit.
       In other words, group names must match one of these patterns:

         ^[_A-Za-z][_A-Za-z0-9]*\z   when PCRE2_UTF is not set
         ^[_\p{L}][_\p{L}\p{Nd}]*\z  when PCRE2_UTF is set

       References  to  capture groups from other parts of the pattern, such as
       backreferences, recursion, and conditions, can all be made by  name  as
       well as by number.

       Named capture groups are allocated numbers as well as names, exactly as
       if  the  names were not present. In both PCRE2 and Perl, capture groups
       are primarily identified by numbers; any names  are  just  aliases  for
       these numbers. The PCRE2 API provides function calls for extracting the
       complete  name-to-number  translation table from a compiled pattern, as
       well as convenience functions for  extracting  captured  substrings  by
       name.

       Warning:  When  more than one capture group has the same number, as de-
       scribed in the previous section, a name given to one of them applies to
       all of them. Perl allows identically numbered groups to have  different
       names.  Consider this pattern, where there are two capture groups, both
       numbered 1:

         (?|(?<AA>aa)|(?<BB>bb))

       Perl  allows  this,  with  both  names AA and BB as aliases of group 1.
       Thus, after a successful match, both names yield the same value (either
       "aa" or "bb").

       In an attempt to reduce confusion, PCRE2 does not allow the same  group
       number to be associated with more than one name. The example above pro-
       vokes  a  compile-time  error. However, there is still scope for confu-
       sion. Consider this pattern:

         (?|(?<AA>aa)|(bb))

       Although the second group number 1 is not explicitly named, the name AA
       is still an alias for any group 1. Whether the pattern matches "aa"  or
       "bb", a reference by name to group AA yields the matched string.

       By  default, a name must be unique within a pattern, except that dupli-
       cate names are permitted for groups with the same number, for example:

         (?|(?<AA>aa)|(?<AA>bb))

       The duplicate name constraint can be disabled by setting the PCRE2_DUP-
       NAMES option at compile time, or by the use of (?J) within the pattern,
       as described in the section entitled "Internal Option Setting" above.

       Duplicate names can be useful for patterns where only one  instance  of
       the  named  capture group can match. Suppose you want to match the name
       of a weekday, either as a 3-letter abbreviation or as  the  full  name,
       and  in  both  cases you want to extract the abbreviation. This pattern
       (ignoring the line breaks) does the job:

         (?J)
         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There are five capture groups, but only one is ever set after a  match.
       The  convenience  functions for extracting the data by name returns the
       substring for the first (and in this example, the only) group  of  that
       name that matched. This saves searching to find which numbered group it
       was.  (An  alternative  way of solving this problem is to use a "branch
       reset" group, as described in the previous section.)

       If you make a backreference to a non-unique named group from  elsewhere
       in  the pattern, the groups to which the name refers are checked in the
       order in which they appear in the overall pattern. The first  one  that
       is  set  is  used  for the reference. For example, this pattern matches
       both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?J)(?:(?<n>foo)|(?<n>bar))\k<n>


       If you make a subroutine call to a non-unique named group, the one that
       corresponds to the first occurrence of the name is used. In the absence
       of duplicate numbers this is the one with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a capture group has matched,
       or to check for recursion, all groups with the same name are tested. If
       the condition is true for any one of them,  the  overall  condition  is
       true.  This is the same behaviour as testing by number. For further de-
       tails of the interfaces for handling  named  capture  groups,  see  the
       pcre2api documentation.


REPETITION

       Repetition  is  specified  by  quantifiers, which may follow any one of
       these items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \R escape sequence
         the \X escape sequence
         any escape sequence that matches a single character
         a character class
         a backreference
         a parenthesized group (including lookaround assertions)
         a subroutine call (recursive or otherwise)

       If a quantifier does not follow a repeatable item, an error occurs. The
       general repetition quantifier specifies a minimum and maximum number of
       permitted matches by giving two numbers  in  curly  brackets  (braces),
       separated  by  a  comma.  The  numbers must be less than 65536, and the
       first must be less than or equal to the second. For example,

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, whereas

         \d{8}

       matches exactly 8 digits. If the first number  is  omitted,  the  lower
       limit is taken as zero; in this case the upper limit must be present.

         X{,4} is interpreted as X{0,4}

       This  is  a  change in behaviour that happened in Perl 5.34.0 and PCRE2
       10.43. In earlier versions such a sequence was  not  interpreted  as  a
       quantifier. Other regular expression engines may behave either way.

       If  the characters that follow an opening brace do not match the syntax
       of a quantifier, the brace is taken as a literal character. In particu-
       lar, this means that {,} is a literal string of three characters.

       Note that not every opening brace is potentially the start of a quanti-
       fier because braces are used  in  other  items  such  as  \N{U+345}  or
       \k{name}.

       In UTF modes, quantifiers apply to characters rather than to individual
       code  units. Thus, for example, \x{100}{2} matches two characters, each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended grapheme clusters, each  of
       which  may  be  several  code  units long (and they may be of different
       lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for capture groups that are referenced as  subroutines  from  else-
       where  in the pattern (but see also the section entitled "Defining cap-
       ture groups for use by reference only" below). Except for parenthesized
       groups, items that have a {0} quantifier are omitted from the  compiled
       pattern.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by following  a  group  that
       can  match no characters with a quantifier that has no upper limit, for
       example:

         (a?)*

       Earlier versions of Perl and PCRE1 used to give  an  error  at  compile
       time for such patterns. However, because there are cases where this can
       be useful, such patterns are now accepted, but whenever an iteration of
       such  a group matches no characters, matching moves on to the next item
       in the pattern instead of repeatedly matching  an  empty  string.  This
       does  not  prevent  backtracking into any of the iterations if a subse-
       quent item fails to match.

       By default, quantifiers are "greedy", that is, they match  as  much  as
       possible  (up  to the maximum number of permitted repetitions), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear  between  /*  and  */ and within the comment, individual * and /
       characters may appear. An attempt to match C comments by  applying  the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails,  because it matches the entire string owing to the greediness of
       the .*  item. However, if a quantifier is followed by a question  mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

         /\*.*?\*/

       does  the right thing with C comments. The meaning of the various quan-
       tifiers is not otherwise changed, just the preferred number of matches.
       Do not confuse this use of question mark with its use as  a  quantifier
       in  its  own  right.   Because it has two uses, it can sometimes appear
       doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE2_UNGREEDY option is set (an option that is not available in
       Perl), the quantifiers are not greedy by default, but  individual  ones
       can  be  made  greedy  by following them with a question mark. In other
       words, it inverts the default behaviour.

       When a parenthesized group is quantified with a  minimum  repeat  count
       that  is  greater  than 1 or with a limited maximum, more memory is re-
       quired for the compiled pattern, in proportion to the size of the mini-
       mum or maximum.

       If a pattern starts with  .*  or  .{0,}  and  the  PCRE2_DOTALL  option
       (equivalent  to  Perl's /s) is set, thus allowing the dot to match new-
       lines, the pattern is implicitly  anchored,  because  whatever  follows
       will  be  tried against every character position in the subject string,
       so there is no point in retrying the overall match at any position  af-
       ter  the  first. PCRE2 normally treats such a pattern as though it were
       preceded by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is worth setting PCRE2_DOTALL in order to obtain this opti-
       mization, or alternatively, using ^ to indicate anchoring explicitly.

       However, there are some cases where the optimization  cannot  be  used.
       When  .*   is  inside  capturing  parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start  may  fail
       where a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is not applied is when the  lead-
       ing  .* is inside an atomic group. Once again, a match at the start may
       fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking  con-
       trol  verbs  (*PRUNE)  and  (*SKIP) also disable this optimization, and
       there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.

       When a capture group is repeated, the value captured is  the  substring
       that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is  "tweedledee". However, if there are nested capture groups, the cor-
       responding captured values may have been set  in  previous  iterations.
       For example, after

         (a|(b))+

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the  repeated  item
       to  be  re-evaluated to see if a different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to  prevent  this,
       either  to  change the nature of the match, or to cause it fail earlier
       than it otherwise might, when the author of the pattern knows there  is
       no point in carrying on.

       Consider,  for  example, the pattern \d+foo when applied to the subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the means for specifying that once a group has matched, it is not to be
       re-evaluated in this way.

       If  we  use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo" the first time.  The  notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       Perl  5.28  introduced an experimental alphabetic form starting with (*
       which may be easier to remember:

         (*atomic:\d+)foo

       This kind of parenthesized group "locks up" the part of the pattern  it
       contains once it has matched, and a failure further into the pattern is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a group of this type matches exactly
       the string of characters that an  identical  standalone  pattern  would
       match, if anchored at the current point in the subject string.

       Atomic  groups  are  not capture groups. Simple cases such as the above
       example can be thought of as a  maximizing  repeat  that  must  swallow
       everything  it can.  So, while both \d+ and \d+? are prepared to adjust
       the number of digits they match in order to make the rest of  the  pat-
       tern match, (?>\d+) can only match an entire sequence of digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       expressions, and can be nested. However, when the contents of an atomic
       group is just a single repeated item, as in the example above,  a  sim-
       pler  notation, called a "possessive quantifier" can be used. This con-
       sists of an additional + character following a quantifier.  Using  this
       notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers are always greedy; the setting of the PCRE2_UN-
       GREEDY option is ignored. They are a convenient notation for  the  sim-
       pler  forms  of  atomic  group.  However, there is no difference in the
       meaning of a possessive quantifier and  the  equivalent  atomic  group,
       though  there  may  be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl  5.8  syn-
       tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It found  its
       way into Perl at release 5.10.

       PCRE2  has  an  optimization  that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when B must follow.  This feature can be disabled by the PCRE2_NO_AUTO-
       POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).

       When a pattern contains an unlimited repeat inside a group that can it-
       self be repeated an unlimited number of times, the  use  of  an  atomic
       group  is the only way to avoid some failing matches taking a very long
       time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all have to be tried. (The  ex-
       ample uses [!?] rather than a single character at the end, because both
       PCRE2 and Perl have an optimization that allows for fast failure when a
       single  character is used. They remember the last single character that
       is required for a match, and fail early if it is  not  present  in  the
       string.)  If  the  pattern  is changed so that it uses an atomic group,
       like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACKREFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a backreference to a  capture  group
       earlier (that is, to its left) in the pattern, provided there have been
       that many previous capture groups.

       However,  if the decimal number following the backslash is less than 8,
       it is always taken as a backreference, and  causes  an  error  only  if
       there  are not that many capture groups in the entire pattern. In other
       words, the group that is referenced need not be to the left of the ref-
       erence for numbers less than 8. A "forward backreference" of this  type
       can make sense when a repetition is involved and the group to the right
       has participated in an earlier iteration.

       It  is  not  possible  to have a numerical "forward backreference" to a
       group whose number is 8 or more using this syntax  because  a  sequence
       such  as  \50  is  interpreted as a character defined in octal. See the
       subsection entitled "Non-printing characters" above for further details
       of the handling of digits following a backslash. Other forms  of  back-
       referencing  do  not suffer from this restriction. In particular, there
       is no problem when named capture groups are used (see below).

       Another way of avoiding the ambiguity inherent in  the  use  of  digits
       following  a  backslash  is  to use the \g escape sequence. This escape
       must be followed by a signed or unsigned number, optionally enclosed in
       braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An unsigned number specifies an absolute reference without the  ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits  follow  the reference. A signed number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the capture group whose number is
       one less than the number of the next group to be started,  so  in  this
       example  (where the next group would be numbered 3) is it equivalent to
       \2, and \g{-2} would be equivalent to \1. Note that if  this  construct
       is  inside  a capture group, that group is included in the count, so in
       this example \g{-2} also refers to group 1:

         (A)(\g{-2}B)

       The use of relative references can be helpful  in  long  patterns,  and
       also  in  patterns  that are created by joining together fragments that
       contain references within themselves.

       The sequence \g{+1} is a reference to the next capture  group  that  is
       started  after  this item, and \g{+2} refers to the one after that, and
       so on. This kind of forward reference can be useful  in  patterns  that
       repeat. Perl does not support the use of + in this way.

       A  backreference  matches  whatever  actually most recently matched the
       capture group in the current subject string, rather  than  anything  at
       all that matches the group (see "Groups as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of  the backreference, the case of letters is relevant. For exam-
       ple,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capture group is matched caselessly.

       There  are  several  different  ways of writing backreferences to named
       capture groups. The .NET syntax  is  \k{name},  the  Python  syntax  is
       (?=name),  and the original Perl syntax is \k<name> or \k'name'. All of
       these are now supported by both Perl and  PCRE2.  Perl  5.10's  unified
       backreference  syntax,  in  which  \g  can be used for both numeric and
       named references, is also supported by PCRE2.   We  could  rewrite  the
       above example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A  capture  group  that is referenced by name may appear in the pattern
       before or after the reference.

       There may be more than one backreference to the same group. If a  group
       has  not actually been used in a particular match, backreferences to it
       always fail by default. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than  "bc".  However,  if
       the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
       erence to an unset value matches an empty string.

       Because  there may be many capture groups in a pattern, all digits fol-
       lowing a backslash are taken as part of a potential backreference  num-
       ber.  If  the  pattern continues with a digit character, some delimiter
       must be used to terminate the backreference. If the  PCRE2_EXTENDED  or
       PCRE2_EXTENDED_MORE  option is set, this can be white space. Otherwise,
       the \g{} syntax or an empty comment (see "Comments" below) can be used.

   Recursive backreferences

       A backreference that occurs inside the group to which it  refers  fails
       when  the  group  is  first used, so, for example, (a\1) never matches.
       However, such references can be useful inside repeated groups. For  ex-
       ample, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the group, the backreference matches the character string cor-
       responding  to  the  previous iteration. In order for this to work, the
       pattern must be such that the first iteration does not  need  to  match
       the  backreference. This can be done using alternation, as in the exam-
       ple above, or by a quantifier with a minimum of zero.

       For versions of PCRE2 less than 10.25, backreferences of this type used
       to cause the group that they reference  to  be  treated  as  an  atomic
       group.   This restriction no longer applies, and backtracking into such
       groups can occur as normal.


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current matching point that does not consume any characters. The simple
       assertions  coded  as  \b,  \B,  \A,  \G, \Z, \z, ^ and $ are described
       above.

       More complicated assertions are coded as  parenthesized  groups.  There
       are  two  kinds:  those  that look ahead of the current position in the
       subject string, and those that look behind it, and in each case an  as-
       sertion  may  be  positive (must match for the assertion to be true) or
       negative (must not match for the assertion to be  true).  An  assertion
       group is matched in the normal way, and if it is true, matching contin-
       ues  after it, but with the matching position in the subject string re-
       set to what it was before the assertion was processed.

       The Perl-compatible lookaround assertions are atomic. If  an  assertion
       is  true, but there is a subsequent matching failure, there is no back-
       tracking into the assertion. However, there are some cases  where  non-
       atomic  assertions can be useful. PCRE2 has some support for these, de-
       scribed in the section entitled "Non-atomic assertions" below, but they
       are not Perl-compatible.

       A lookaround assertion may appear as the  condition  in  a  conditional
       group  (see  below). In this case, the result of matching the assertion
       determines which branch of the condition is followed.

       Assertion groups are not capture groups. If an assertion contains  cap-
       ture  groups within it, these are counted for the purposes of numbering
       the capture groups in the whole pattern. Within each branch of  an  as-
       sertion,  locally  captured  substrings  may be referenced in the usual
       way. For example, a sequence such as (.)\g{-1} can  be  used  to  check
       that two adjacent characters are the same.

       When  a  branch within an assertion fails to match, any substrings that
       were captured are discarded (as happens with any  pattern  branch  that
       fails  to  match).  A  negative  assertion  is  true  only when all its
       branches fail to match; this means that no captured substrings are ever
       retained after a successful negative assertion. When an assertion  con-
       tains a matching branch, what happens depends on the type of assertion.

       For  a  positive  assertion, internally captured substrings in the suc-
       cessful branch are retained, and matching continues with the next  pat-
       tern  item  after  the  assertion. For a negative assertion, a matching
       branch means that the assertion is not true. If such  an  assertion  is
       being  used as a condition in a conditional group (see below), captured
       substrings are retained,  because  matching  continues  with  the  "no"
       branch of the condition. For other failing negative assertions, control
       passes to the previous backtracking point, thus discarding any captured
       strings within the assertion.

       Most  assertion groups may be repeated; though it makes no sense to as-
       sert the same thing several times, the side effect of capturing in pos-
       itive assertions may occasionally be useful. However, an assertion that
       forms the condition for a conditional  group  may  not  be  quantified.
       PCRE2  used  to restrict the repetition of assertions, but from release
       10.35 the only restriction is that an unlimited maximum  repetition  is
       changed  to  be one more than the minimum. For example, {3,} is treated
       as {3,4}.

   Alphabetic assertion names

       Traditionally, symbolic sequences such as (?= and (?<= have  been  used
       to  specify lookaround assertions. Perl 5.28 introduced some experimen-
       tal alphabetic alternatives which might be easier to remember. They all
       start with (* instead of (? and must be written using lower  case  let-
       ters. PCRE2 supports the following synonyms:

         (*positive_lookahead:  or (*pla: is the same as (?=
         (*negative_lookahead:  or (*nla: is the same as (?!
         (*positive_lookbehind: or (*plb: is the same as (?<=
         (*negative_lookbehind: or (*nlb: is the same as (?<!

       For  example,  (*pla:foo) is the same assertion as (?=foo). In the fol-
       lowing sections, the various assertions are described using the  origi-
       nal symbolic forms.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way to do it is with (?!) because an empty string al-
       ways matches, so an assertion that requires there not to  be  an  empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind  assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does find an occurrence of "bar" that is not  preceded  by  "foo".  The
       contents  of a lookbehind assertion are restricted such that there must
       be a known maximum to the lengths of all the strings it matches.  There
       are two cases:

       If every top-level alternative matches a fixed length, for example

         (?<=colour|color)

       there  is a limit of 65535 characters to the lengths, which do not have
       to be the same, as this example demonstrates. This is the only kind  of
       lookbehind  supported  by  PCRE2 versions earlier than 10.43 and by the
       alternative matching function pcre2_dfa_match().

       In PCRE2 10.43 and later, pcre2_match() supports lookbehind  assertions
       in  which  one  or  more top-level alternatives can match more than one
       string length, for example

         (?<=colou?r)

       The maximum matching length for any branch of the lookbehind is limited
       to a value set by the calling program (default 255 characters).  Unlim-
       ited  repetition (for example \d*) is not supported. In some cases, the
       escape sequence \K (see above) can be used instead of a lookbehind  as-
       sertion  at  the  start  of a pattern to get round the length limit re-
       striction.

       In UTF-8 and UTF-16 modes, PCRE2 does not allow the  \C  escape  (which
       matches  a single code unit even in a UTF mode) to appear in lookbehind
       assertions, because it makes it impossible to calculate the  length  of
       the  lookbehind.  The \X and \R escapes, which can match different num-
       bers of code units, are never permitted in lookbehinds.

       "Subroutine" calls (see below) such as (?2) or (?&X) are  permitted  in
       lookbehinds,  as  long  as  the called capture group matches a limited-
       length string. However, recursion, that is, a "subroutine" call into  a
       group that is already active, is not supported.

       PCRE2  supports backreferences in lookbehinds, but only if certain con-
       ditions are met. The PCRE2_MATCH_UNSET_BACKREF option must not be  set,
       there  must be no use of (?| in the pattern (it creates duplicate group
       numbers), and if the backreference is by name, the name must be unique.
       Of course, the referenced group must itself match a limited length sub-
       string. The following pattern matches words  containing  at  least  two
       characters that begin and end with the same character:

          \b(\w)\w++(?<=\1)

       Possessive  quantifiers  can be used in conjunction with lookbehind as-
       sertions to specify efficient matching at the end of  subject  strings.
       Consider a simple pattern such as

         abcd$

       when  applied  to  a  long string that does not match. Because matching
       proceeds from left to right, PCRE2 will look for each "a" in  the  sub-
       ject  and  then see if what follows matches the rest of the pattern. If
       the pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this  fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item because of the possessive
       quantifier; it can match only the entire string. The subsequent lookbe-
       hind  assertion  does  a single test on the last four characters. If it
       fails, the match fails immediately. For  long  strings,  this  approach
       makes a significant difference to the processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This  time  the  first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


NON-ATOMIC ASSERTIONS

       Traditional lookaround assertions are atomic. That is, if an  assertion
       is  true, but there is a subsequent matching failure, there is no back-
       tracking into the assertion. However, there are some cases  where  non-
       atomic  positive  assertions  can be useful. PCRE2 provides these using
       the following syntax:

         (*non_atomic_positive_lookahead:  or (*napla: or (?*
         (*non_atomic_positive_lookbehind: or (*naplb: or (?<*

       Consider the problem of finding the right-most word in  a  string  that
       also  appears  earlier  in the string, that is, it must appear at least
       twice in total.  This pattern returns the required result  as  captured
       substring 1:

         ^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}

       For  a subject such as "word1 word2 word3 word2 word3 word4" the result
       is "word3". How does it work? At the start, ^(?x) anchors  the  pattern
       and sets the "x" option, which causes white space (introduced for read-
       ability)  to  be  ignored. Inside the assertion, the greedy .* at first
       consumes the entire string, but then has to backtrack until the rest of
       the assertion can match a word, which is captured by group 1. In  other
       words,  when  the  assertion first succeeds, it captures the right-most
       word in the string.

       The current matching point is then reset to the start of  the  subject,
       and  the  rest  of  the pattern match checks for two occurrences of the
       captured word, using an ungreedy .*? to scan from  the  left.  If  this
       succeeds,  we are done, but if the last word in the string does not oc-
       cur twice, this part of the pattern  fails.  If  a  traditional  atomic
       lookahead  (?=  or (*pla: had been used, the assertion could not be re-
       entered, and the whole match would fail. The pattern would succeed only
       if the very last word in the subject was found twice.

       Using a non-atomic lookahead, however, means that when  the  last  word
       does  not  occur  twice  in the string, the lookahead can backtrack and
       find the second-last word, and so on, until either the match  succeeds,
       or all words have been tested.

       Two conditions must be met for a non-atomic assertion to be useful: the
       contents  of one or more capturing groups must change after a backtrack
       into the assertion, and there must be  a  backreference  to  a  changed
       group  later  in  the pattern. If this is not the case, the rest of the
       pattern match fails exactly as before because nothing has  changed,  so
       using a non-atomic assertion just wastes resources.

       There  is one exception to backtracking into a non-atomic assertion. If
       an (*ACCEPT) control verb is triggered, the assertion  succeeds  atomi-
       cally.  That  is,  a subsequent match failure cannot backtrack into the
       assertion.

       Non-atomic assertions are not supported  by  the  alternative  matching
       function pcre2_dfa_match(). They are supported by JIT, but only if they
       do not contain any control verbs such as (*ACCEPT). (This may change in
       future). Note that assertions that appear as conditions for conditional
       groups (see below) must be atomic.


SCRIPT RUNS

       In  concept, a script run is a sequence of characters that are all from
       the same Unicode script such as Latin or Greek. However,  because  some
       scripts  are  commonly  used together, and because some diacritical and
       other marks are used with multiple scripts,  it  is  not  that  simple.
       There is a full description of the rules that PCRE2 uses in the section
       entitled "Script Runs" in the pcre2unicode documentation.

       If  part  of a pattern is enclosed between (*script_run: or (*sr: and a
       closing parenthesis, it fails if the sequence  of  characters  that  it
       matches  are not a script run. After a failure, normal backtracking oc-
       curs. Script runs can be used to detect spoofing attacks using  charac-
       ters  that  look  the  same, but are from different scripts. The string
       "paypal.com" is an infamous example, where the letters could be a  mix-
       ture of Latin and Cyrillic. This pattern ensures that the matched char-
       acters in a sequence of non-spaces that follow white space are a script
       run:

         \s+(*sr:\S+)

       To  be  sure  that  they are all from the Latin script (for example), a
       lookahead can be used:

         \s+(?=\p{Latin})(*sr:\S+)

       This works as long as the first character is expected to be a character
       in that script, and not (for example)  punctuation,  which  is  allowed
       with  any script. If this is not the case, a more creative lookahead is
       needed. For example, if digits, underscore, and dots are  permitted  at
       the start:

         \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)


       In  many  cases, backtracking into a script run pattern fragment is not
       desirable. The script run can employ an atomic group to  prevent  this.
       Because  this is a common requirement, a shorthand notation is provided
       by (*atomic_script_run: or (*asr:

         (*asr:...) is the same as (*sr:(?>...))

       Note that the atomic group is inside the script run. Putting it outside
       would not prevent backtracking into the script run pattern.

       Support for script runs is not available if PCRE2 is  compiled  without
       Unicode support. A compile-time error is given if any of the above con-
       structs  is encountered. Script runs are not supported by the alternate
       matching function, pcre2_dfa_match() because they use the  same  mecha-
       nism as capturing parentheses.

       Warning:  The  (*ACCEPT)  control  verb  (see below) should not be used
       within a script run group, because it causes an immediate exit from the
       group, bypassing the script run checking.


CONDITIONAL GROUPS

       It is possible to cause the matching process to obey a pattern fragment
       conditionally or to choose between two alternative fragments, depending
       on the result of an assertion, or whether a specific capture group  has
       already been matched. The two possible forms of conditional group are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. An absent no-pattern is equivalent  to
       an  empty string (it always matches). If there are more than two alter-
       natives in the group, a compile-time error occurs. Each of the two  al-
       ternatives may itself contain nested groups of any form, including con-
       ditional  groups;  the  restriction to two alternatives applies only at
       the level of the condition itself. This pattern fragment is an  example
       where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are five kinds of condition: references to capture groups, refer-
       ences  to  recursion,  two pseudo-conditions called DEFINE and VERSION,
       and assertions.

   Checking for a used capture group by number

       If the text between the parentheses consists of a sequence  of  digits,
       the  condition is true if a capture group of that number has previously
       matched. If there is more than one capture group with the  same  number
       (see  the earlier section about duplicate group numbers), the condition
       is true if any of them have matched. An alternative notation, which  is
       a PCRE2 extension, not supported by Perl, is to precede the digits with
       a plus or minus sign. In this case, the group number is relative rather
       than  absolute.  The most recently opened capture group (which could be
       enclosing this condition) can be referenced by (?(-1),  the  next  most
       recent by (?(-2), and so on. Inside loops it can also make sense to re-
       fer  to  subsequent groups.  The next capture group to be opened can be
       referenced as (?(+1), and so on. The value zero in any of  these  forms
       is not used; it provokes a compile-time error.

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE2_EXTENDED  option)  and
       to divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond part matches one or more characters that are not  parentheses.  The
       third  part  is a conditional group that tests whether or not the first
       capture group matched. If it did, that is, if subject started  with  an
       opening  parenthesis,  the condition is true, and so the yes-pattern is
       executed and a closing parenthesis is required.  Otherwise,  since  no-
       pattern is not present, the conditional group matches nothing. In other
       words,  this  pattern matches a sequence of non-parentheses, optionally
       enclosed in parentheses.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used capture group by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  capture group by name. For compatibility with earlier versions of
       PCRE1, which had this facility before Perl, the syntax (?(name)...)  is
       also  recognized.   Note, however, that undelimited names consisting of
       the letter R followed by digits are ambiguous (see the  following  sec-
       tion). Rewriting the above example to use a named group gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If  the  name used in a condition of this kind is a duplicate, the test
       is applied to all groups of the same name, and is true if  any  one  of
       them has matched.

   Checking for pattern recursion

       "Recursion"  in  this sense refers to any subroutine-like call from one
       part of the pattern to another, whether or not it  is  actually  recur-
       sive.  See  the  sections  entitled "Recursive patterns" and "Groups as
       subroutines" below for details of recursion and subroutine calls.

       If a condition is the string (R), and there is no  capture  group  with
       the  name R, the condition is true if matching is currently in a recur-
       sion or subroutine call to the whole pattern or any capture  group.  If
       digits  follow  the letter R, and there is no group with that name, the
       condition is true if the most recent call is  into  a  group  with  the
       given  number,  which must exist somewhere in the overall pattern. This
       is a contrived example that is equivalent to a+b:

         ((?(R1)a+|(?1)b))

       However, in both cases, if there is a capture  group  with  a  matching
       name,  the  condition tests for its being set, as described in the sec-
       tion above, instead of testing for recursion. For example,  creating  a
       group  with  the  name  R1  by adding (?<R1>) to the above pattern com-
       pletely changes its meaning.

       If a name preceded by ampersand follows the letter R, for example:

         (?(R&name)...)

       the condition is true if the most recent recursion is into a  group  of
       that name (which must exist within the pattern).

       This condition does not check the entire recursion stack. It tests only
       the  current  level.  If the name used in a condition of this kind is a
       duplicate, the test is applied to all groups of the same name,  and  is
       true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false.

   Defining capture groups for use by reference only

       If the condition is the string (DEFINE), the condition is always false,
       even  if there is a group with the name DEFINE. In this case, there may
       be only one alternative in the rest of the conditional group. It is al-
       ways skipped if control reaches this point in the pattern; the idea  of
       DEFINE  is that it can be used to define subroutines that can be refer-
       enced from elsewhere. (The use of subroutines is described below.)  For
       example,  a  pattern  to match an IPv4 address such as "192.168.23.245"
       could be written like this (ignore white space and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group  inside  which  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, insist-
       ing on a word boundary at each end.

   Checking the PCRE2 version

       Programs that link with a PCRE2 library can check the version by  call-
       ing  pcre2_config()  with  appropriate arguments. Users of applications
       that do not have access to the underlying code cannot do this.  A  spe-
       cial  "condition" called VERSION exists to allow such users to discover
       which version of PCRE2 they are dealing with by using this condition to
       match a string such as "yesno". VERSION must be followed either by  "="
       or ">=" and a version number.  For example:

         (?(VERSION>=10.4)yes|no)

       This  pattern matches "yes" if the PCRE2 version is greater or equal to
       10.4, or "no" otherwise. The fractional part of the version number  may
       not contain more than two digits.

   Assertion conditions

       If  the  condition  is  not  in  any of the above formats, it must be a
       parenthesized assertion. This may be a positive or  negative  lookahead
       or  lookbehind  assertion. However, it must be a traditional atomic as-
       sertion, not one of the non-atomic assertions.

       Consider this pattern, again containing  non-significant  white  space,
       and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition  is  a  positive lookahead assertion that matches an op-
       tional sequence of non-letters followed by a letter. In other words, it
       tests for the presence of at least one letter in the subject. If a let-
       ter is found, the subject is matched  against  the  first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.

       When an assertion that is a condition contains capture groups, any cap-
       turing  that  occurs  in  a matching branch is retained afterwards, for
       both positive and negative assertions, because matching always  contin-
       ues  after  the  assertion, whether it succeeds or fails. (Compare non-
       conditional assertions, for which captures are retained only for  posi-
       tive assertions that succeed.)


COMMENTS

       There are two ways of including comments in patterns that are processed
       by  PCRE2.  In  both  cases,  the start of the comment must not be in a
       character class, nor in the middle of any  other  sequence  of  related
       characters  such  as (?: or a group name or number. The characters that
       make up a comment play no part in the pattern matching.

       The sequence (?# marks the start of a comment that continues up to  the
       next  closing parenthesis. Nested parentheses are not permitted. If the
       PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is  set,  an  unescaped  #
       character  also  introduces  a comment, which in this case continues to
       immediately after the next newline character or character  sequence  in
       the pattern. Which characters are interpreted as newlines is controlled
       by  an option passed to the compiling function or by a special sequence
       at the start of the pattern, as described in the section entitled "New-
       line conventions" above. Note that the end of this type of comment is a
       literal newline sequence in the pattern; escape sequences  that  happen
       to represent a newline do not count. For example, consider this pattern
       when  PCRE2_EXTENDED is set, and the default newline convention (a sin-
       gle linefeed character) is in force:

         abc #comment \n still comment

       On encountering the # character, pcre2_compile() skips  along,  looking
       for  a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an  actual  character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider  the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of  recursion,  the  best
       that  can  be  done  is  to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to  handle  an  arbitrary  nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously,  PCRE2  cannot  support  the interpolation of Perl code. In-
       stead, it supports special syntax for recursion of the entire  pattern,
       and also for individual capture group recursion. After its introduction
       in PCRE1 and Python, this kind of recursion was subsequently introduced
       into Perl at release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine  call  of  the
       capture  group of the given number, provided that it occurs inside that
       group. (If not, it is a non-recursive subroutine  call,  which  is  de-
       scribed in the next section.) The special item (?R) or (?0) is a recur-
       sive call of the entire regular expression.

       This  PCRE2  pattern  solves the nested parentheses problem (assume the
       PCRE2_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which can either be a sequence of non-parentheses, or a re-
       cursive match of the pattern itself (that is, a correctly parenthesized
       substring).  Finally there is a closing parenthesis. Note the use of  a
       possessive  quantifier  to  avoid  backtracking  into sequences of non-
       parentheses.

       If this were part of a larger pattern, you would not  want  to  recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We  have  put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
       tricky.  This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most recently opened parentheses  preceding  the  recursion.  In  other
       words,  a  negative  number counts capturing parentheses leftwards from
       the point at which it is encountered.

       Be aware however, that if duplicate capture group numbers are  in  use,
       relative  references  refer  to the earliest group with the appropriate
       number. Consider, for example:

         (?|(a)|(b)) (c) (?-2)

       The first two capture groups (a) and (b) are both numbered 1, and group
       (c) is number 2. When the reference (?-2) is  encountered,  the  second
       most  recently opened parentheses has the number 1, but it is the first
       such group (the (a) group) to which the recursion refers. This would be
       the same if an absolute reference (?1) was used. In other words,  rela-
       tive references are just a shorthand for computing a group number.

       It  is  also possible to refer to subsequent capture groups, by writing
       references such as (?+2). However, these cannot  be  recursive  because
       the  reference  is not inside the parentheses that are referenced. They
       are always non-recursive subroutine calls, as  described  in  the  next
       section.

       An  alternative  approach  is to use named parentheses. The Perl syntax
       for this is (?&name); PCRE1's earlier syntax  (?P>name)  is  also  sup-
       ported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one group with the same name, the earliest one is
       used.

       The example pattern that we have been looking at contains nested unlim-
       ited  repeats,  and  so the use of a possessive quantifier for matching
       strings of non-parentheses is important when applying  the  pattern  to
       strings that do not match. For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields  "no  match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there  are
       so  many  different  ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing  parentheses  are  those
       from  the outermost level. If you want to obtain intermediate values, a
       callout function can be used (see below and the pcre2callout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the value for the inner capturing parentheses  (numbered  2)  is  "ef",
       which  is  the last value taken on at the top level. If a capture group
       is not matched at the top level, its final  captured  value  is  unset,
       even  if it was (temporarily) set at a deeper level during the matching
       process.

       Do not confuse the (?R) item with the condition (R),  which  tests  for
       recursion.   Consider  this pattern, which matches text in angle brack-
       ets, allowing for arbitrary nesting. Only digits are allowed in  nested
       brackets  (that is, when recursing), whereas any characters are permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional  group,  with  two
       different  alternatives  for the recursive and non-recursive cases. The
       (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE2 and Perl

       Some former differences between PCRE2 and Perl no longer exist.

       Before release 10.30, recursion processing in PCRE2 differed from  Perl
       in  that  a  recursive  subroutine call was always treated as an atomic
       group. That is, once it had matched some of the subject string, it  was
       never  re-entered,  even if it contained untried alternatives and there
       was a subsequent matching failure. (Historical note:  PCRE  implemented
       recursion before Perl did.)

       Starting  with  release 10.30, recursive subroutine calls are no longer
       treated as atomic. That is, they can be re-entered to try unused alter-
       natives if there is a matching failure later in the  pattern.  This  is
       now  compatible  with the way Perl works. If you want a subroutine call
       to be atomic, you must explicitly enclose it in an atomic group.

       Supporting backtracking into recursions simplifies certain types of re-
       cursive pattern. For example, this pattern matches palindromic strings:

         ^((.)(?1)\2|.?)$

       The second branch in the group matches a single  central  character  in
       the  palindrome  when there are an odd number of characters, or nothing
       when there are an even number of characters, but in order  to  work  it
       has  to  be  able  to  try the second case when the rest of the pattern
       match fails. If you want to match typical palindromic phrases, the pat-
       tern has to ignore all non-word characters,  which  can  be  done  like
       this:

         ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$

       If  run  with  the  PCRE2_CASELESS option, this pattern matches phrases
       such as "A man, a plan, a canal: Panama!". Note the use of the  posses-
       sive  quantifier  *+  to  avoid backtracking into sequences of non-word
       characters. Without this, PCRE2 takes a great deal longer (ten times or
       more) to match typical phrases, and Perl takes so long that  you  think
       it has gone into a loop.

       Another  way  in which PCRE2 and Perl used to differ in their recursion
       processing is in the handling of captured  values.  Formerly  in  Perl,
       when  a  group  was called recursively or as a subroutine (see the next
       section), it had no access to any values that were captured outside the
       recursion, whereas in PCRE2 these values can  be  referenced.  Consider
       this pattern:

         ^(.)(\1|a(?2))

       This  pattern matches "bab". The first capturing parentheses match "b",
       then in the second group, when the backreference \1 fails to match "b",
       the second alternative matches "a" and then recurses. In the recursion,
       \1 does now match "b" and so the whole match succeeds. This match  used
       to fail in Perl, but in later versions (I tried 5.024) it now works.


GROUPS AS SUBROUTINES

       If  the syntax for a recursive group call (either by number or by name)
       is used outside the parentheses to which it refers, it operates  a  bit
       like  a  subroutine  in  a programming language. More accurately, PCRE2
       treats the referenced group as an independent subpattern which it tries
       to match at the current matching position. The called group may be  de-
       fined  before  or  after the reference. A numbered reference can be ab-
       solute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Another example is  given  in  the  discussion  of  DEFINE
       above.

       Like  recursions,  subroutine  calls  used to be treated as atomic, but
       this changed at PCRE2 release 10.30, so  backtracking  into  subroutine
       calls  can  now  occur. However, any capturing parentheses that are set
       during the subroutine call revert to their previous values afterwards.

       Processing options such as case-independence are fixed when a group  is
       defined,  so  if  it  is  used  as a subroutine, such options cannot be
       changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the  change  of
       processing option does not affect the called group.

       The  behaviour  of  backtracking control verbs in groups when called as
       subroutines is described in the section entitled "Backtracking verbs in
       subroutines" below.


ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for calling a group as a subroutine, possibly re-
       cursively.  Here  are  two  of the examples used above, rewritten using
       this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE2 supports an extension to Oniguruma: if a number is preceded by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a backreference; the latter is  a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl  code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE2 provides a similar feature, but of course it  cannot  obey  arbi-
       trary  Perl  code. The feature is called "callout". The caller of PCRE2
       provides an external function by putting its entry  point  in  a  match
       context  using  the function pcre2_set_callout(), and then passing that
       context to pcre2_match() or pcre2_dfa_match(). If no match  context  is
       passed, or if the callout entry point is set to NULL, callouts are dis-
       abled.

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There  are  two  kinds  of  callout:
       those  with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A  numerical  argument
       allows  the  application  to  distinguish  between  different callouts.
       String arguments were added for release 10.20 to make it  possible  for
       script  languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external func-
       tion is called. It is provided with the number or  string  argument  of
       the  callout, the position in the pattern, and one item of data that is
       also set in the match block. The callout function may cause matching to
       proceed, to backtrack, or to fail.

       By default, PCRE2 implements a  number  of  optimizations  at  matching
       time,  and  one  side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options  that
       disable  the relevant optimizations. More details, including a complete
       description of the programming interface to the callout  function,  are
       given in the pcre2callout documentation.

   Callouts with numerical arguments

       If  you  just  want  to  have  a means of identifying different callout
       points, put a number less than 256 after the  letter  C.  For  example,
       this pattern has two callout points:

         (?C1)abc(?C2)def

       If  the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in  the  pattern.
       They  are all numbered 255. If there is a conditional group in the pat-
       tern whose condition is an assertion, an additional callout is inserted
       just before the condition. An explicit callout may also be set at  this
       position, as in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

   Callouts with string arguments

       A  delimited  string may be used instead of a number as a callout argu-
       ment. The starting delimiter must be one of ` ' " ^ % #  $  {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing  delimiter  is  }.  If  the  ending  delimiter is needed within the
       string, it must be doubled. For example:

         (?C'ab ''c'' d')xyz(?C{any text})pqr

       The doubling is removed before the string  is  passed  to  the  callout
       function.


BACKTRACKING CONTROL

       There  are  a  number  of  special "Backtracking Control Verbs" (to use
       Perl's terminology) that modify the behaviour  of  backtracking  during
       matching.  They are generally of the form (*VERB) or (*VERB:NAME). Some
       verbs take either form, and may behave differently depending on whether
       or not a name argument is present. The names are  not  required  to  be
       unique within the pattern.

       By  default,  for  compatibility  with  Perl, a name is any sequence of
       characters that does not include a closing parenthesis. The name is not
       processed in any way, and it is  not  possible  to  include  a  closing
       parenthesis   in  the  name.   This  can  be  changed  by  setting  the
       PCRE2_ALT_VERBNAMES option, but the result is no  longer  Perl-compati-
       ble.

       When  PCRE2_ALT_VERBNAMES  is  set,  backslash processing is applied to
       verb names and only an unescaped  closing  parenthesis  terminates  the
       name.  However, the only backslash items that are permitted are \Q, \E,
       and sequences such as \x{100} that define character code points.  Char-
       acter type escapes such as \d are faulted.

       A closing parenthesis can be included in a name either as \) or between
       \Q  and  \E. In addition to backslash processing, if the PCRE2_EXTENDED
       or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb
       names is skipped, and #-comments are recognized, exactly as in the rest
       of the pattern.  PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do  not  affect
       verb names unless PCRE2_ALT_VERBNAMES is also set.

       The  maximum  length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if  the
       closing  parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a pat-
       tern. Except for (*ACCEPT), they may not be quantified.

       Since these verbs are specifically related  to  backtracking,  most  of
       them  can be used only when the pattern is to be matched using the tra-
       ditional matching function, because that uses a backtracking algorithm.
       With the exception of (*FAIL), which behaves like  a  failing  negative
       assertion, the backtracking control verbs cause an error if encountered
       by the DFA matching function.

       The  behaviour  of  these  verbs in repeated groups, assertions, and in
       capture groups called as subroutines (whether or  not  recursively)  is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may  know  the minimum length of matching subject, or that a particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by  setting  the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
       pile(), or by starting the pattern with (*NO_START_OPT). There is  more
       discussion of this option in the section entitled "Compiling a pattern"
       in the pcre2api documentation.

       Experiments  with  Perl  suggest that it too has similar optimizations,
       and like PCRE2, turning them off can change the result of a match.

   Verbs that act immediately

       The following verbs act as soon as they are encountered.

          (*ACCEPT) or (*ACCEPT:NAME)

       This verb causes the match to end successfully, skipping the  remainder
       of  the  pattern.  However,  when  it is inside a capture group that is
       called as a subroutine, only that group is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive assertion, the assertion succeeds; in a  negative  assertion,  the
       assertion fails.

       If  (*ACCEPT)  is inside capturing parentheses, the data so far is cap-
       tured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B"  is  cap-
       tured by the outer parentheses.

       (*ACCEPT)  is  the only backtracking verb that is allowed to be quanti-
       fied because an ungreedy quantification with a  minimum  of  zero  acts
       only when a backtrack happens. Consider, for example,

         (A(*ACCEPT)??B)C

       where  A,  B, and C may be complex expressions. After matching "A", the
       matcher processes "BC"; if that fails, causing a  backtrack,  (*ACCEPT)
       is  triggered  and the match succeeds. In both cases, all but C is cap-
       tured. Whereas (*COMMIT) (see below) means "fail on backtrack",  a  re-
       peated (*ACCEPT) of this type means "succeed on backtrack".

       Warning:  (*ACCEPT)  should  not be used within a script run group, be-
       cause it causes an immediate exit from the group, bypassing the  script
       run checking.

         (*FAIL) or (*FAIL:NAME)

       This  verb causes a matching failure, forcing backtracking to occur. It
       may be abbreviated to (*F). It is equivalent  to  (?!)  but  easier  to
       read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that
       are  not  present  in PCRE2. The nearest equivalent is the callout fea-
       ture, as for example in this pattern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

       (*ACCEPT:NAME)  and  (*FAIL:NAME)  behave the same as (*MARK:NAME)(*AC-
       CEPT) and (*MARK:NAME)(*FAIL), respectively,  that  is,  a  (*MARK)  is
       recorded just before the verb acts.

   Recording which path was taken

       There  is  one  verb whose main purpose is to track how a match was ar-
       rived at, though it also has a secondary use in  conjunction  with  ad-
       vancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A  name is always required with this verb. For all the other backtrack-
       ing control verbs, a NAME argument is optional.

       When a match succeeds, the name of the last-encountered  mark  name  on
       the matching path is passed back to the caller as described in the sec-
       tion entitled "Other information about the match" in the pcre2api docu-
       mentation.  This  applies  to all instances of (*MARK) and other verbs,
       including those inside assertions and atomic groups. However, there are
       differences in those cases when (*MARK) is  used  in  conjunction  with
       (*SKIP) as described below.

       The  mark name that was last encountered on the matching path is passed
       back. A verb without a NAME argument is ignored for this purpose.  Here
       is  an  example of pcre2test output, where the "mark" modifier requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple it indicates which of the two alternatives matched. This is a  more
       efficient  way of obtaining this information than putting each alterna-
       tive in its own capturing parentheses.

       If a verb with a name is encountered in a positive  assertion  that  is
       true,  the  name  is recorded and passed back if it is the last-encoun-
       tered. This does not happen for negative assertions or failing positive
       assertions.

       After a partial match or a failed match, the last encountered  name  in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XP
         No match, mark = B

       Note  that  in  this  unanchored  example the mark is retained from the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in  (*MARK)  values  after  failed  matches,  you
       should  probably  set the PCRE2_NO_START_OPTIMIZE option (see above) to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is a subsequent  match  failure,
       causing  a  backtrack  to the verb, a failure is forced. That is, back-
       tracking cannot pass to the left of the  verb.  However,  when  one  of
       these verbs appears inside an atomic group or in a lookaround assertion
       that  is  true,  its effect is confined to that group, because once the
       group has been matched, there is never any backtracking into it.  Back-
       tracking from beyond an assertion or an atomic group ignores the entire
       group, and seeks a preceding backtracking point.

       These  verbs  differ  in exactly what kind of failure occurs when back-
       tracking reaches them. The behaviour described below  is  what  happens
       when  the  verb is not in a subroutine or an assertion. Subsequent sec-
       tions cover these special cases.

         (*COMMIT) or (*COMMIT:NAME)

       This verb causes the whole match to fail outright if there is  a  later
       matching failure that causes backtracking to reach it. Even if the pat-
       tern  is  unanchored,  no further attempts to find a match by advancing
       the starting point take place. If (*COMMIT) is  the  only  backtracking
       verb that is encountered, once it has been passed pcre2_match() is com-
       mitted to finding a match at the current starting point, or not at all.
       For example:

         a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish."

       The behaviour of (*COMMIT:NAME) is not the same  as  (*MARK:NAME)(*COM-
       MIT).  It is like (*MARK:NAME) in that the name is remembered for pass-
       ing back to the caller. However, (*SKIP:NAME) searches only  for  names
       that are set with (*MARK), ignoring those set by any of the other back-
       tracking verbs.

       If  there  is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first,  so  merely  passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an an-
       chor,  unless  PCRE2's  start-of-match optimizations are turned off, as
       shown in this output from pcre2test:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data>
         re> /(*COMMIT)abc/no_start_optimize
         data> xyzabc
         No match

       For the first pattern, PCRE2 knows that any match must start with  "a",
       so  the optimization skips along the subject to "a" before applying the
       pattern to the first set of data. The match attempt then succeeds.  The
       second  pattern disables the optimization that skips along to the first
       character. The pattern is now applied  starting  at  "x",  and  so  the
       (*COMMIT)  causes  the  match to fail without trying any other starting
       points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position  in
       the subject if there is a later matching failure that causes backtrack-
       ing  to  reach it. If the pattern is unanchored, the normal "bumpalong"
       advance to the next starting character then happens.  Backtracking  can
       occur  as  usual to the left of (*PRUNE), before it is reached, or when
       matching to the right of (*PRUNE), but if there  is  no  match  to  the
       right,  backtracking cannot cross (*PRUNE). In simple cases, the use of
       (*PRUNE) is just an alternative to an atomic group or possessive  quan-
       tifier, but there are some uses of (*PRUNE) that cannot be expressed in
       any  other  way. In an anchored pattern (*PRUNE) has the same effect as
       (*COMMIT).

       The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to the caller. However, (*SKIP:NAME) searches only for names  set  with
       (*MARK), ignoring those set by other backtracking verbs.

         (*SKIP)

       This  verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered.  (*SKIP)  signifies that whatever text was matched leading up to
       it cannot be part of a successful match if there is a  later  mismatch.
       Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifier does not have the same effect as this example; although it would
       suppress  backtracking  during  the first match attempt, the second at-
       tempt would start at the second character instead  of  skipping  on  to
       "c".

       If  (*SKIP) is used to specify a new starting position that is the same
       as the starting position of the current match, or (by  being  inside  a
       lookbehind)  earlier, the position specified by (*SKIP) is ignored, and
       instead the normal "bumpalong" occurs.

         (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour  is  modified.  When
       such  a  (*SKIP) is triggered, the previous path through the pattern is
       searched for the most recent (*MARK) that has the same name. If one  is
       found,  the  "bumpalong" advance is to the subject position that corre-
       sponds to that (*MARK) instead of to where (*SKIP) was encountered.  If
       no (*MARK) with a matching name is found, the (*SKIP) is ignored.

       The  search  for a (*MARK) name uses the normal backtracking mechanism,
       which means that it does not  see  (*MARK)  settings  that  are  inside
       atomic groups or assertions, because they are never re-entered by back-
       tracking. Compare the following pcre2test examples:

           re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: a
          1: a
         data:
           re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: b
          1: b

       In  the first example, the (*MARK) setting is in an atomic group, so it
       is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
       This allows the second branch of the pattern to be tried at  the  first
       character  position.  In the second example, the (*MARK) setting is not
       in an atomic group. This allows (*SKIP:X) to find the (*MARK)  when  it
       backtracks, and this causes a new matching attempt to start at the sec-
       ond  character.  This  time, the (*MARK) is never seen because "a" does
       not match "b", so the matcher immediately jumps to the second branch of
       the pattern.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME).  It
       ignores names that are set by other backtracking verbs.

         (*THEN) or (*THEN:NAME)

       This  verb  causes  a skip to the next innermost alternative when back-
       tracking reaches it. That  is,  it  cancels  any  further  backtracking
       within  the  current  alternative.  Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further  items
       after  the  end  of the group if FOO succeeds); on failure, the matcher
       skips to the second alternative and tries COND2,  without  backtracking
       into  COND1.  If that succeeds and BAR fails, COND3 is tried. If subse-
       quently BAZ fails, there are no more alternatives, so there is a  back-
       track  to  whatever came before the entire group. If (*THEN) is not in-
       side an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is not the same  as  (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       A group that does not contain a | character is just a part of  the  en-
       closing  alternative;  it is not a nested alternation with only one al-
       ternative. The effect of (*THEN) extends beyond such a group to the en-
       closing alternative.  Consider this pattern, where A, B, etc. are  com-
       plex  pattern  fragments  that  do not contain any | characters at this
       level:

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does  not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However,  if  the  group containing (*THEN) is given an alternative, it
       behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner group. After a fail-
       ure in C, matching moves to (*FAIL), which causes the  whole  group  to
       fail  because  there  are  no  more  alternatives to try. In this case,
       matching does backtrack into A.

       Note that a conditional group is not considered as having two  alterna-
       tives,  because  only one is ever used. In other words, the | character
       in a conditional group has a different meaning. Ignoring  white  space,
       consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*? is un-
       greedy,  it initially matches zero characters. The condition (?=a) then
       fails, the character "b" is matched, but "c" is  not.  At  this  point,
       matching  does  not  backtrack to .*? as might perhaps be expected from
       the presence of the | character. The conditional group is part  of  the
       single  alternative  that comprises the whole pattern, and so the match
       fails. (If there was a backtrack into .*?, allowing it  to  match  "b",
       the match would succeed.)

       The  verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing  the  match
       at  the  current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except  that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb

       If  more  than  one  backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example,  consider  this  pat-
       tern, where A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If  A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried.  This  behaviour
       is  consistent,  but is not always the same as Perl's. It means that if
       two or more backtracking verbs appear in succession, all but  the  last
       of them has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes  it to be triggered, and its action is taken. There can never be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE2 sometimes differs from Perl in its handling of backtracking verbs
       in repeated groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches  unless  its  optimizations  are
       disabled,  but  PCRE2  always fails because the (*COMMIT) in the second
       repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL) in any assertion has its normal effect: it forces an  immediate
       backtrack.  The  behaviour  of  the other backtracking verbs depends on
       whether or not the assertion is standalone or acting as  the  condition
       in a conditional group.

       (*ACCEPT)  in  a  standalone positive assertion causes the assertion to
       succeed without any further processing; captured  strings  and  a  mark
       name  (if  set) are retained. In a standalone negative assertion, (*AC-
       CEPT) causes the assertion to fail without any further processing; cap-
       tured substrings and any mark name are discarded.

       If the assertion is a condition, (*ACCEPT) causes the condition  to  be
       true  for  a  positive assertion and false for a negative one; captured
       substrings are retained in both cases.

       The remaining verbs act only when a later failure causes a backtrack to
       reach them. This means that, for the Perl-compatible assertions,  their
       effect is confined to the assertion, because Perl lookaround assertions
       are atomic. A backtrack that occurs after such an assertion is complete
       does  not  jump  back  into  the  assertion.  Note in particular that a
       (*MARK) name that is set in an assertion is not "seen" by  an  instance
       of (*SKIP:NAME) later in the pattern.

       PCRE2  now supports non-atomic positive assertions, as described in the
       section entitled "Non-atomic assertions" above. These  assertions  must
       be  standalone  (not used as conditions). They are not Perl-compatible.
       For these assertions, a later backtrack does jump back into the  asser-
       tion,  and  therefore verbs such as (*COMMIT) can be triggered by back-
       tracks from later in the pattern.

       The effect of (*THEN) is not allowed to escape beyond an assertion.  If
       there  are no more branches to try, (*THEN) causes a positive assertion
       to be false, and a negative assertion to be true.

       The other backtracking verbs are not treated specially if  they  appear
       in  a  standalone  positive assertion. In a conditional positive asser-
       tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
       or (*PRUNE) causes the condition to be false. However, for both  stand-
       alone and conditional negative assertions, backtracking into (*COMMIT),
       (*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
       ing any further alternative branches.

   Backtracking verbs in subroutines

       These behaviours occur whether or not the group is called recursively.

       (*ACCEPT) in a group called as a subroutine causes the subroutine match
       to  succeed without any further processing. Matching then continues af-
       ter the subroutine call. Perl documents this behaviour.  Perl's  treat-
       ment of the other verbs in subroutines is different in some cases.

       (*FAIL)  in  a  group  called as a subroutine has its normal effect: it
       forces an immediate backtrack.

       (*COMMIT), (*SKIP), and (*PRUNE) cause the  subroutine  match  to  fail
       when  triggered  by being backtracked to in a group called as a subrou-
       tine. There is then a backtrack at the outer level.

       (*THEN), when triggered, skips to the next alternative in the innermost
       enclosing group that has alternatives (its normal behaviour).  However,
       if there is no such group within the subroutine's group, the subroutine
       match fails and there is a backtrack at the outer level.


SEE ALSO

       pcre2api(3),    pcre2callout(3),    pcre2matching(3),   pcre2syntax(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 04 June 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.44                      04 June 2024                  PCRE2PATTERN(3)
------------------------------------------------------------------------------



PCRE2PERFORM(3)            Library Functions Manual            PCRE2PERFORM(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE2 into a reasonably efficient interpretive
       code,  so  that most simple patterns do not use much memory for storing
       the compiled version. However, there is one case where the memory usage
       of a compiled pattern can be unexpectedly  large.  If  a  parenthesized
       group  has  a quantifier with a minimum greater than 1 and/or a limited
       maximum, the whole group is repeated in the compiled code. For example,
       the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack  points  within
       each of the repetitions can be independently maintained.)

       For  regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large,  and  par-
       ticularly  if  such repetitions are nested, the memory usage can become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses over 50KiB when compiled using the 8-bit library.  When  PCRE2  is
       compiled  with its default internal pointer size of two bytes, the size
       limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit
       libraries, and this is reached with the above pattern if the outer rep-
       etition is increased from 3 to 4. PCRE2 can be compiled to  use  larger
       internal  pointers  and thus handle larger compiled patterns, but it is
       better to try to rewrite your pattern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to  make  use
       of PCRE2's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces  the memory requirements to around 16KiB, and indeed it remains
       under 20KiB even with the outer repetition increased to  100.  However,
       this kind of pattern is not always exactly equivalent, because any cap-
       tures  within  subroutine calls are lost when the subroutine completes.
       If this is not a problem, this kind of  rewriting  will  allow  you  to
       process  patterns that PCRE2 cannot otherwise handle. The matching per-
       formance of the two different versions of the pattern are  roughly  the
       same.  (This applies from release 10.30 - things were different in ear-
       lier releases.)


STACK AND HEAP USAGE AT RUN TIME

       From release 10.30, the interpretive (non-JIT) version of pcre2_match()
       uses very little system stack at run time. In earlier  releases  recur-
       sive  function  calls  could  use a great deal of stack, and this could
       cause problems, but this usage has been eliminated. Backtracking  posi-
       tions  are now explicitly remembered in memory frames controlled by the
       code.

       The size of each frame depends on the size of pointer variables and the
       number of capturing parenthesized groups in the pattern being  matched.
       On a 64-bit system the frame size for a pattern with no captures is 128
       bytes. For each capturing group the size increases by 16 bytes.

       Until  release  10.41,  an initial 20KiB frames vector was allocated on
       the system stack, but this still caused some  issues  for  multi-thread
       applications  where  each  thread  has a very small stack. From release
       10.41 backtracking memory frames are always held  in  heap  memory.  An
       initial heap allocation is obtained the first time any match data block
       is  passed  to  pcre2_match().  This  is remembered with the match data
       block and re-used if that block is used for another match. It is  freed
       when the match data block itself is freed.

       The  size  of the initial block is the larger of 20KiB or ten times the
       pattern's frame size, unless the heap limit is less than this, in which
       case the heap limit is used. If the initial  block  proves  to  be  too
       small during matching, it is replaced by a larger block, subject to the
       heap  limit.  The  heap limit is checked only when a new block is to be
       allocated. Reducing the heap limit between calls to pcre2_match()  with
       the same match data block does not affect the saved block.

       In  contrast  to  pcre2_match(),  pcre2_dfa_match()  does use recursive
       function calls, but only for processing atomic groups,  lookaround  as-
       sertions, and recursion within the pattern. The original version of the
       code  used  to  allocate  quite large internal workspace vectors on the
       stack, which caused some problems for  some  patterns  in  environments
       with  small  stacks.  From release 10.32 the code for pcre2_dfa_match()
       has been re-factored to use heap memory  when  necessary  for  internal
       workspace  when  recursing,  though  recursive function calls are still
       used.

       The "match depth" parameter can be used to limit the depth of  function
       recursion,  and  the  "match  heap"  parameter  to limit heap memory in
       pcre2_dfa_match().


PROCESSING TIME

       Certain items in regular expression patterns are processed  more  effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]   than   a   set   of  single-character  alternatives  such  as
       (a|e|i|o|u). In general, the simplest construction  that  provides  the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains  a  lot  of useful general discussion about optimizing regular
       expressions for efficient performance. This document contains a few ob-
       servations about PCRE2.

       Using Unicode character properties (the \p,  \P,  and  \X  escapes)  is
       slow,  because  PCRE2 has to use a multi-stage table lookup whenever it
       needs a character's property. If you can find  an  alternative  pattern
       that does not use character properties, it will probably be faster.

       By  default,  the  escape  sequences  \b, \d, \s, and \w, and the POSIX
       character classes such as [:alpha:]  do  not  use  Unicode  properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you  can  set  the PCRE2_UCP option or start the pattern with
       (*UCP) if you want Unicode character properties to be  used.  This  can
       double  the  matching  time  for  items  such  as \d, when matched with
       pcre2_match(); the performance loss is less with a DFA  matching  func-
       tion, and in both cases there is not much difference for \b.

       When  a pattern begins with .* not in atomic parentheses, nor in paren-
       theses that are the subject of a backreference,  and  the  PCRE2_DOTALL
       option  is  set,  the pattern is implicitly anchored by PCRE2, since it
       can match only at the start of a subject string.  If  the  pattern  has
       multiple top-level branches, they must all be anchorable. The optimiza-
       tion  can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is au-
       tomatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If PCRE2_DOTALL is not set, PCRE2 cannot make  this  optimization,  be-
       cause  the  dot metacharacter does not then match a newline, and if the
       subject string contains newlines, the pattern may match from the  char-
       acter immediately following one of them instead of from the very start.
       For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to  do  this, PCRE2 has to retry the match starting after every newline
       in the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain   newlines,   the   best   performance   is  obtained  by  setting
       PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate  ex-
       plicit  anchoring.  That saves PCRE2 from having to scan along the sub-
       ject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail,  PCRE2  has
       in  principle to try every possible variation, and this can take an ex-
       tremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure, PCRE2 checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier. This can often reduce  mem-
       ory requirements as well. As another example, consider this pattern:

         ([^<]|<(?!inet))+

       It  matches  from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that  might  be  used  when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a parenthesis is processed,  a  backtracking
       position  is  passed,  so this formulation uses a memory frame for each
       matched character. For a long string, a lot of memory is required. Con-
       sider now this  rewritten  pattern,  which  matches  exactly  the  same
       strings:

         ([^<]++|<(?!inet))+

       This runs much faster, because sequences of characters that do not con-
       tain "<" are "swallowed" in one item inside the parentheses, and a pos-
       sessive  quantifier  is  used to stop any backtracking into the runs of
       non-"<" characters. This version also uses a lot  less  memory  because
       entry  to  a  new  set of parentheses happens only when a "<" character
       that is not followed by "inet" is encountered (and we  assume  this  is
       relatively rare).

       This example shows that one way of optimizing performance when matching
       long  subject strings is to write repeated parenthesized subpatterns to
       match more than one character whenever possible.

   SETTING RESOURCE LIMITS

       You can set limits on the amount of processing that  takes  place  when
       matching,  and  on  the amount of heap memory that is used. The default
       values of the limits are very large, and unlikely ever to operate. They
       can be changed when PCRE2 is built, and  they  can  also  be  set  when
       pcre2_match()  or pcre2_dfa_match() is called. For details of these in-
       terfaces, see the pcre2build documentation  and  the  section  entitled
       "The match context" in the pcre2api documentation.

       The  pcre2test  test program has a modifier called "find_limits" which,
       if applied to a subject line, causes it to  find  the  smallest  limits
       that allow a pattern to match. This is done by repeatedly matching with
       different limits.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 27 July 2022
       Copyright (c) 1997-2022 University of Cambridge.


PCRE2 10.41                      27 July 2022                  PCRE2PERFORM(3)
------------------------------------------------------------------------------



PCRE2POSIX(3)              Library Functions Manual              PCRE2POSIX(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SYNOPSIS

       #include <pcre2posix.h>

       int pcre2_regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int pcre2_regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t pcre2_regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void pcre2_regfree(regex_t *preg);


DESCRIPTION

       This  set of functions provides a POSIX-style API for the PCRE2 regular
       expression 8-bit library. There are no POSIX-style wrappers for PCRE2's
       16-bit and 32-bit libraries. See the pcre2api documentation for  a  de-
       scription  of  PCRE2's native API, which contains much additional func-
       tionality.

       IMPORTANT NOTE: The functions described here are NOT  thread-safe,  and
       should  not  be used in multi-threaded applications. They are also lim-
       ited to processing subjects that are not bigger than 2GB. Use  the  na-
       tive API instead.

       These  functions  are  wrapper functions that ultimately call the PCRE2
       native API. Their prototypes are defined  in  the  pcre2posix.h  header
       file, and they all have unique names starting with pcre2_. However, the
       pcre2posix.h  header  also  contains macro definitions that convert the
       standard POSIX names such  regcomp()  into  pcre2_regcomp()  etc.  This
       means  that a program can use the usual POSIX names without running the
       risk of accidentally linking with POSIX functions from a different  li-
       brary.

       On  Unix-like systems the PCRE2 POSIX library is called libpcre2-posix,
       so can be accessed by adding -lpcre2-posix to the command  for  linking
       an application. Because the POSIX functions call the native ones, it is
       also necessary to add -lpcre2-8.

       On Windows systems, if you are linking to a DLL version of the library,
       it  is  recommended  that PCRE2POSIX_SHARED is defined before including
       the pcre2posix.h header, as it will allow for a more efficient  way  to
       invoke the functions by adding the __declspec(dllimport) decorator.

       Although  they were not defined as prototypes in pcre2posix.h, releases
       10.33 to 10.36 of the library contained functions with the POSIX  names
       regcomp()  etc.  These simply passed their arguments to the PCRE2 func-
       tions. These functions were provided for backwards  compatibility  with
       earlier  versions  of  PCRE2, which had only POSIX names. However, this
       has proved troublesome in situations where a program links with several
       libraries, some of which use PCRE2's POSIX interface while  others  use
       the  real  POSIX functions.  For this reason, the POSIX names have been
       removed since release 10.37.

       Calling the header file pcre2posix.h avoids  any  conflict  with  other
       POSIX  libraries.  It can, of course, be renamed or aliased as regex.h,
       which is the "correct" name, if there is  no  clash.  It  provides  two
       structure  types,  regex_t  for compiled internal forms, and regmatch_t
       for returning captured substrings. It also defines some constants whose
       names start with "REG_"; these are used for setting options and identi-
       fying error codes.


USING THE POSIX FUNCTIONS

       Note that these functions are just POSIX-style wrappers for PCRE2's na-
       tive API.  They do not give POSIX  regular  expression  behaviour,  and
       they are not thread-safe or even POSIX compatible.

       Those  POSIX  option bits that can reasonably be mapped to PCRE2 native
       options have been implemented. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot in PCRE2 as a replacement library. Other POSIX options
       are not even defined.

       There are also some options that are not defined by POSIX.  These  have
       been  added  at  the  request  of users who want to make use of certain
       PCRE2-specific features via the POSIX calling interface or to  add  BSD
       or GNU functionality.

       When  PCRE2  is  called via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE2 options, as described below. "POSIX-like in style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-unit encoding  domains  it  is  probably
       even less compatible.

       The  descriptions  below use the actual names of the functions, but, as
       described above, the standard POSIX names (without the  pcre2_  prefix)
       may also be used.


COMPILING A PATTERN

       The function pcre2_regcomp() is called to compile a pattern into an in-
       ternal  form. By default, the pattern is a C string terminated by a bi-
       nary zero (but see REG_PEND below). The preg argument is a pointer to a
       regex_t structure that is used as a base for storing information  about
       the  compiled  regular  expression.  It  is  also  used  for input when
       REG_PEND is set. The regex_t structure used by pcre2_regcomp()  is  de-
       fined  in  pcre2posix.h  and  is  not the same as the structure used by
       other libraries that provide POSIX-style matching.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The PCRE2_DOTALL option is set when the regular  expression  is  passed
       for  compilation  to  the  native function. Note that REG_DOTALL is not
       part of the POSIX standard.

         REG_ICASE

       The PCRE2_CASELESS option is set when the regular expression is  passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSPEC

       The  PCRE2_LITERAL  option is set when the regular expression is passed
       for compilation to the native function. This disables all meta  charac-
       ters  in the pattern, causing it to be treated as a literal string. The
       only other options that are  allowed  with  REG_NOSPEC  are  REG_ICASE,
       REG_NOSUB,  REG_PEND,  and REG_UTF. Note that REG_NOSPEC is not part of
       the POSIX standard.

         REG_NOSUB

       When  a  pattern  that  is  compiled  with  this  flag  is  passed   to
       pcre2_regexec()  for  matching, the nmatch and pmatch arguments are ig-
       nored, and no captured strings are returned. Versions of the  PCRE  li-
       brary  prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile op-
       tion, but this no longer happens because it disables the use  of  back-
       references.

         REG_PEND

       If  this option is set, the reg_endp field in the preg structure (which
       has the type const char *) must be set to point to the character beyond
       the end of the pattern before calling pcre2_regcomp(). The pattern  it-
       self  may  now  contain binary zeros, which are treated as data charac-
       ters. Without REG_PEND, a binary zero terminates the  pattern  and  the
       re_endp field is ignored. This is a GNU extension to the POSIX standard
       and  should be used with caution in software intended to be portable to
       other systems.

         REG_UCP

       The PCRE2_UCP option is set when the regular expression is  passed  for
       compilation  to  the  native function. This causes PCRE2 to use Unicode
       properties when matching \d, \w,  etc.,  instead  of  just  recognizing
       ASCII values. Note that REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The  PCRE2_UNGREEDY option is set when the regular expression is passed
       for compilation to the native function. Note that REG_UNGREEDY  is  not
       part of the POSIX standard.

         REG_UTF

       The  PCRE2_UTF  option is set when the regular expression is passed for
       compilation to the native function. This causes the pattern itself  and
       all  data  strings used for matching it to be treated as UTF-8 strings.
       Note that REG_UTF is not part of the POSIX standard.

       In the absence of these flags, no options  are  passed  to  the  native
       function.  This means that the regex is compiled with PCRE2 default se-
       mantics.  In  particular,  the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way.  Note  that  setting
       PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE.
       It  does not affect the way newlines are matched by the dot metacharac-
       ter (they are not) or by a negative class such as [^a] (they are).

       The yield of pcre2_regcomp() is zero on success,  and  non-zero  other-
       wise.  The preg structure is filled in on success, and one other member
       of  the  structure (as well as re_endp) is public: re_nsub contains the
       number of capturing subpatterns in the regular expression. Various  er-
       ror codes are defined in the header file.

       NOTE: If the yield of pcre2_regcomp() is non-zero, you must not attempt
       to use the contents of the preg structure. If, for example, you pass it
       to  pcre2_regexec(), the result is undefined and your program is likely
       to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE2 to obey POSIX  semantics,  but
       then PCRE2 was never intended to be a POSIX engine. The following table
       lists  the  different  possibilities for matching newline characters in
       Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This behaviour is not what happens when PCRE2 is called via  its  POSIX
       API.  By  default, PCRE2's behaviour is the same as Perl's, except that
       there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both  PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default  POSIX newline handling can be obtained by setting PCRE2_DOTALL
       and PCRE2_DOLLAR_ENDONLY when  calling  pcre2_compile()  directly,  but
       there is no way to make PCRE2 behave exactly as for the REG_NEWLINE ac-
       tion.  When  using  the  POSIX  API,  passing  REG_NEWLINE  to  PCRE2's
       pcre2_regcomp()  function  causes  PCRE2_MULTILINE  to  be  passed   to
       pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to
       pass PCRE2_DOLLAR_ENDONLY.


MATCHING A PATTERN

       The function pcre2_regexec() is called to match a compiled pattern preg
       against  a  given string, which is by default terminated by a zero byte
       (but see REG_STARTEND below), subject to the options in eflags.   These
       can be:

         REG_NOTBOL

       The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_NOTEMPTY

       The  PCRE2_NOTEMPTY  option  is  set  when calling the underlying PCRE2
       matching function. Note that REG_NOTEMPTY is  not  part  of  the  POSIX
       standard.  However, setting this option can give more POSIX-like behav-
       iour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_STARTEND

       When this option  is  set,  the  subject  string  starts  at  string  +
       pmatch[0].rm_so  and  ends  at  string  + pmatch[0].rm_eo, which should
       point to the first character beyond the string. There may be binary ze-
       ros within the subject string, and indeed, using  REG_STARTEND  is  the
       only way to pass a subject string that contains a binary zero.

       Whatever  the  value  of  pmatch[0].rm_so,  the  offsets of the matched
       string and any captured substrings are  still  given  relative  to  the
       start  of  string  itself. (Before PCRE2 release 10.30 these were given
       relative to string + pmatch[0].rm_so, but this differs from  other  im-
       plementations.)

       This  is  a  BSD  extension,  compatible with but not specified by IEEE
       Standard 1003.2 (POSIX.2), and should be used with caution in  software
       intended  to  be  portable to other systems. Note that a non-zero rm_so
       does not imply REG_NOTBOL; REG_STARTEND affects only the  location  and
       length  of  the string, not how it is matched. Setting REG_STARTEND and
       passing pmatch as NULL are mutually exclusive; the error REG_INVARG  is
       returned.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       pcre2_regexec()  are  ignored  (except  possibly as input for REG_STAR-
       TEND).

       The value of nmatch may be zero, and the value pmatch may be NULL  (un-
       less  REG_STARTEND  is  set);  in  both  these  cases no data about any
       matched strings is returned.

       Otherwise, the portion of the string that was  matched,  and  also  any
       captured substrings, are returned via the pmatch argument, which points
       to  an  array  of  nmatch structures of type regmatch_t, containing the
       members rm_so and rm_eo. These contain the byte  offset  to  the  first
       character of each substring and the offset to the first character after
       the  end of each substring, respectively. The 0th element of the vector
       relates to the entire portion of string that  was  matched;  subsequent
       elements relate to the capturing subpatterns of the regular expression.
       Unused entries in the array have both structure members set to -1.

       regmatch_t  as  well  as  the  regoff_t  typedef it uses are defined in
       pcre2posix.h and are not warranted to have the same size or  layout  as
       other  similarly  named  types from other libraries that provide POSIX-
       style matching.

       A successful match yields a zero return; various error  codes  are  de-
       fined  in the header file, of which REG_NOMATCH is the "expected" fail-
       ure code.


ERROR MESSAGES

       The pcre2_regerror() function maps a  non-zero  errorcode  from  either
       pcre2_regcomp()  or  pcre2_regexec() to a printable message. If preg is
       not NULL, the error should have arisen from the use of that  structure.
       A  message  terminated  by  a  binary  zero is placed in errbuf. If the
       buffer is too short, only the first errbuf_size - 1 characters  of  the
       error message are used. The yield of the function is the size of buffer
       needed  to hold the whole message, including the terminating zero. This
       value is greater than errbuf_size if the message was truncated.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with the preg structure. The function pcre2_regfree() frees all
       such memory, after which preg may no longer be used as a  compiled  ex-
       pression.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 19 January 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.43                     19 January 2024                  PCRE2POSIX(3)
------------------------------------------------------------------------------



PCRE2SAMPLE(3)             Library Functions Manual             PCRE2SAMPLE(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 SAMPLE PROGRAM

       A  simple, complete demonstration program to get you started with using
       PCRE2 is supplied in the file pcre2demo.c in the src directory  in  the
       PCRE2 distribution. A listing of this program is given in the pcre2demo
       documentation. If you do not have a copy of the PCRE2 distribution, you
       can save this listing to re-create the contents of pcre2demo.c.

       The  demonstration  program compiles the regular expression that is its
       first argument, and matches it against the subject string in its second
       argument. No PCRE2 options are set, and default  character  tables  are
       used. If matching succeeds, the program outputs the portion of the sub-
       ject  that  matched,  together  with  the contents of any captured sub-
       strings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       The code in pcre2demo.c is an 8-bit program that uses the  PCRE2  8-bit
       library.  It  handles  strings  and characters that are stored in 8-bit
       code units.  By default, one character corresponds to  one  code  unit,
       but  if  the  pattern starts with "(*UTF)", both it and the subject are
       treated as UTF-8 strings, where characters  may  occupy  multiple  code
       units.

       If  PCRE2  is installed in the standard include and library directories
       for your operating system, you should be able to compile the demonstra-
       tion program using a command like this:

         cc -o pcre2demo pcre2demo.c -lpcre2-8

       If PCRE2 is installed elsewhere, you may need to add additional options
       to the command line. For example, on a Unix-like system that has  PCRE2
       installed  in /usr/local, you can compile the demonstration program us-
       ing a command like this:

         cc -o pcre2demo -I/usr/local/include pcre2demo.c \
            -L/usr/local/lib -lpcre2-8

       Once you have built the demonstration program, you can run simple tests
       like this:

         ./pcre2demo 'cat|dog' 'the cat sat on the mat'
         ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'

       Note that there is a  much  more  comprehensive  test  program,  called
       pcre2test,  which supports many more facilities for testing regular ex-
       pressions using all three PCRE2 libraries (8-bit, 16-bit,  and  32-bit,
       though  not all three need be installed). The pcre2demo program is pro-
       vided as a relatively simple coding example.

       If you try to run pcre2demo when PCRE2 is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file
       or directory

       This  is  caused  by the way shared library support works on those sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 February 2016
       Copyright (c) 1997-2016 University of Cambridge.


PCRE2 10.22                    02 February 2016                 PCRE2SAMPLE(3)
------------------------------------------------------------------------------

PCRE2SERIALIZE(3)          Library Functions Manual          PCRE2SERIALIZE(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run. However, if you are using the just-in-time  optimization  feature,
       it is not possible to save and reload the JIT data, because it is posi-
       tion-dependent.  The  host  on  which the patterns are reloaded must be
       running the same version of PCRE2, with the same code unit  width,  and
       must  also have the same endianness, pointer width and PCRE2_SIZE type.
       For example, patterns compiled on a 32-bit system using PCRE2's  16-bit
       library cannot be reloaded on a 64-bit system, nor can they be reloaded
       using the 8-bit library.

       Note  that  "serialization" in PCRE2 does not convert compiled patterns
       to an abstract format like Java or .NET serialization.  The  serialized
       output  is really just a bytecode dump, which is why it can only be re-
       loaded in the same environment as the one that created  it.  Hence  the
       restrictions  mentioned  above.   Applications  that are not statically
       linked with a fixed version of PCRE2 must be prepared to recompile pat-
       terns from their sources, in order to be immune to PCRE2 upgrades.


SECURITY CONCERNS

       The facility for saving and restoring compiled patterns is intended for
       use within individual applications.  As  such,  the  data  supplied  to
       pcre2_serialize_decode()  is expected to be trusted data, not data from
       arbitrary external sources.  There  is  only  some  simple  consistency
       checking, not complete validation of what is being re-loaded. Corrupted
       data may cause undefined results. For example, if the length field of a
       pattern in the serialized data is corrupted, the deserializing code may
       read beyond the end of the byte stream that is passed to it.


SAVING COMPILED PATTERNS

       Before compiled patterns can be saved they must be serialized, which in
       PCRE2  means converting the pattern to a stream of bytes. A single byte
       stream may contain any number of compiled patterns, but they  must  all
       use  the same character tables. A single copy of the tables is included
       in the byte stream (its size is 1088 bytes). For more details of  char-
       acter  tables,  see the section on locale support in the pcre2api docu-
       mentation.

       The function pcre2_serialize_encode() creates a serialized byte  stream
       from  a  list of compiled patterns. Its first two arguments specify the
       list, being a pointer to a vector of pointers to compiled patterns, and
       the length of the vector. The third and fourth arguments point to vari-
       ables which are set to point to the created byte stream and its length,
       respectively. The final argument is a pointer  to  a  general  context,
       which  can  be  used  to specify custom memory management functions. If
       this argument is NULL, malloc() is used to obtain memory for  the  byte
       stream. The yield of the function is the number of serialized patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA      the number of patterns is zero or less
         PCRE2_ERROR_BADMAGIC     mismatch of id bytes in one of the patterns
         PCRE2_ERROR_NOMEMORY     memory allocation failed
         PCRE2_ERROR_MIXEDTABLES  the patterns do not all use the same tables
         PCRE2_ERROR_NULL         the 1st, 3rd, or 4th argument is NULL

       PCRE2_ERROR_BADMAGIC  means  either that a pattern's code has been cor-
       rupted, or that a slot in the vector does not point to a compiled  pat-
       tern.

       Once a set of patterns has been serialized you can save the data in any
       appropriate  manner. Here is sample code that compiles two patterns and
       writes them to a file. It assumes that the variable fd refers to a file
       that is open for output. The error checking that should be present in a
       real application has been omitted for simplicity.

         int errorcode;
         uint8_t *bytes;
         PCRE2_SIZE erroroffset;
         PCRE2_SIZE bytescount;
         pcre2_code *list_of_codes[2];
         list_of_codes[0] = pcre2_compile("first pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         list_of_codes[1] = pcre2_compile("second pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
           &bytescount, NULL);
         errorcode = fwrite(bytes, 1, bytescount, fd);

       Note that the serialized data is binary data that may  contain  any  of
       the  256  possible  byte values. On systems that make a distinction be-
       tween binary and non-binary data, be sure that the file is  opened  for
       binary output.

       Serializing  a  set  of patterns leaves the original data untouched, so
       they can still be used for matching. Their memory  must  eventually  be
       freed in the usual way by calling pcre2_code_free(). When you have fin-
       ished with the byte stream, it too must be freed by calling pcre2_seri-
       alize_free().  If  this function is called with a NULL argument, it re-
       turns immediately without doing anything.


RE-USING PRECOMPILED PATTERNS

       In order to re-use a set of saved patterns you must first make the  se-
       rialized  byte stream available in main memory (for example, by reading
       from a file). The management of this memory block is up to the applica-
       tion. You can use the pcre2_serialize_get_number_of_codes() function to
       find out how many compiled patterns are in the serialized data  without
       actually decoding the patterns:

         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);

       The pcre2_serialize_decode() function reads a byte stream and recreates
       the compiled patterns in new memory blocks, setting pointers to them in
       a  vector.  The  first two arguments are a pointer to a suitable vector
       and its length, and the third argument points to a byte stream. The fi-
       nal argument is a pointer to a general context, which can  be  used  to
       specify custom memory management functions for the decoded patterns. If
       this argument is NULL, malloc() and free() are used. After deserializa-
       tion, the byte stream is no longer needed and can be discarded.

         pcre2_code *list_of_codes[2];
         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes =
           pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);

       If  the  vector  is  not  large enough for all the patterns in the byte
       stream, it is filled with those that fit, and  the  remainder  are  ig-
       nored.  The yield of the function is the number of decoded patterns, or
       one of the following negative error codes:

         PCRE2_ERROR_BADDATA    second argument is zero or less
         PCRE2_ERROR_BADMAGIC   mismatch of id bytes in the data
         PCRE2_ERROR_BADMODE    mismatch of code unit size or PCRE2 version
         PCRE2_ERROR_BADSERIALIZEDDATA  other sanity check failure
         PCRE2_ERROR_MEMORY     memory allocation failed
         PCRE2_ERROR_NULL       first or third argument is NULL

       PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it  was
       compiled on a system with different endianness.

       Decoded patterns can be used for matching in the usual way, and must be
       freed  by  calling pcre2_code_free(). However, be aware that there is a
       potential race issue if you are using multiple patterns that  were  de-
       coded  from a single byte stream in a multithreaded application. A sin-
       gle copy of the character tables is used by all  the  decoded  patterns
       and a reference count is used to arrange for its memory to be automati-
       cally  freed when the last pattern is freed, but there is no locking on
       this reference count. Therefore, if you want to call  pcre2_code_free()
       for  these  patterns  in  different  threads, you must arrange your own
       locking, and ensure that pcre2_code_free()  cannot  be  called  by  two
       threads at the same time.

       If  a pattern was processed by pcre2_jit_compile() before being serial-
       ized, the JIT data is discarded and so is no longer available  after  a
       save/restore  cycle.  You can, however, process a restored pattern with
       pcre2_jit_compile() if you wish.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 27 June 2018
       Copyright (c) 1997-2018 University of Cambridge.


PCRE2 10.32                      27 June 2018                PCRE2SERIALIZE(3)
------------------------------------------------------------------------------



PCRE2SYNTAX(3)             Library Functions Manual             PCRE2SYNTAX(3)


NAME
       PCRE2 - Perl-compatible regular expressions (revised API)


PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE2 are described in the pcre2pattern  documentation.  This
       document contains a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal

       Note that white space inside \Q...\E is always treated as literal, even
       if PCRE2_EXTENDED is set, causing most other white space to be ignored.


BRACED ITEMS

       With  one  exception, wherever brace characters { and } are required to
       enclose data for constructions such as \g{2} or \k{name}, space  and/or
       horizontal  tab  characters  that follow { or precede } are allowed and
       are ignored. In the case of quantifiers, they may also appear before or
       after the comma. The exception is \u{...} which is not  Perl-compatible
       and is recognized only when PCRE2_EXTRA_ALT_BSUX is set. This is an EC-
       MAScript compatibility feature, and follows ECMAScript's behaviour.


ESCAPED CHARACTERS

       This  table  applies to ASCII and Unicode environments. An unrecognized
       escape sequence causes an error.

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is a non-control ASCII character
         \e         escape (hex 1B)
         \f         form feed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \0dd       character with octal code 0dd
         \ddd       character with octal code ddd, or backreference
         \o{ddd..}  character with octal code ddd..
         \N{U+hh..} character with Unicode code point hh.. (Unicode mode only)
         \xhh       character with hex code hh
         \x{hh..}   character with hex code hh..

       If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the
       following are also recognized:

         \U         the character "U"
         \uhhhh     character with hex code hhhh
         \u{hh..}   character with hex code hh.. but only for EXTRA_ALT_BSUX

       When \x is not followed by {, from zero to two hexadecimal  digits  are
       read,  but in ALT_BSUX mode \x must be followed by two hexadecimal dig-
       its to be recognized as a hexadecimal escape; otherwise  it  matches  a
       literal  "x".   Likewise,  if  \u (in ALT_BSUX mode) is not followed by
       four hexadecimal digits or (in EXTRA_ALT_BSUX mode) a sequence  of  hex
       digits in curly brackets, it matches a literal "u".

       Note that \0dd is always an octal code. The treatment of backslash fol-
       lowed  by  a non-zero digit is complicated; for details see the section
       "Non-printing characters" in the pcre2pattern documentation, where  de-
       tails  of  escape  processing  in  EBCDIC  environments are also given.
       \N{U+hh..} is synonymous with \x{hh..} in PCRE2 but is not supported in
       EBCDIC environments. Note that \N not  followed  by  an  opening  curly
       bracket has a different meaning (see below).


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one code unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal white space character
         \H         a character that is not a horizontal white space character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a white space character
         \S         a character that is not a white space character
         \v         a vertical white space character
         \V         a character that is not a vertical white space character
         \w         a "word" character
         \W         a "non-word" character
         \X         a Unicode extended grapheme cluster

       \C  is dangerous because it may leave the current matching point in the
       middle of a UTF-8 or UTF-16 character. The application can lock out the
       use of \C by setting the PCRE2_NEVER_BACKSLASH_C  option.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       By  default,  \d, \s, and \w match only ASCII characters, even in UTF-8
       mode or in the 16-bit and 32-bit libraries. However, if locale-specific
       matching is happening, \s and \w may also match  characters  with  code
       points in the range 128-255. If the PCRE2_UCP option is set, the behav-
       iour of these escape sequences is changed to use Unicode properties and
       they  match  many  more  characters, but there are some option settings
       that can restrict individual sequences to matching only  ASCII  charac-
       ters.

       Property descriptions in \p and \P are matched caselessly; hyphens, un-
       derscores,  and  white  space are ignored, in accordance with Unicode's
       "loose matching" rules.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         Lc         Ll, Lu, or Lt
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, VT, FF, CR
         Xuc        Universally-named character: one that can be
                      represented by a Universal Character Name
         Xwd        Perl word: property Xan or underscore

       Perl and POSIX space are now the same. Perl added VT to its space char-
       acter set at release 5.18.


BINARY PROPERTIES FOR \p AND \P

       Unicode defines a number of  binary  properties,  that  is,  properties
       whose  only  values  are  true or false. You can obtain a list of those
       that are recognized by \p and \P, along with  their  abbreviations,  by
       running this command:

         pcre2test -LP


SCRIPT MATCHING WITH \p AND \P

       Many  script  names  and their 4-letter abbreviations are recognized in
       \p{sc:...} or \p{scx:...} items, or on their own with \p (and  also  \P
       of course). You can obtain a list of these scripts by running this com-
       mand:

         pcre2test -LS


THE BIDI_CLASS PROPERTY FOR \p AND \P

         \p{Bidi_Class:<class>}   matches a character with the given class
         \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

         AL          Arabic letter
         AN          Arabic number
         B           paragraph separator
         BN          boundary neutral
         CS          common separator
         EN          European number
         ES          European separator
         ET          European terminator
         FSI         first strong isolate
         L           left-to-right
         LRE         left-to-right embedding
         LRI         left-to-right isolate
         LRO         left-to-right override
         NSM         non-spacing mark
         ON          other neutral
         PDF         pop directional format
         PDI         pop directional isolate
         R           right-to-left
         RLE         right-to-left embedding
         RLI         right-to-left isolate
         RLO         right-to-left override
         S           segment separator
         WS          which space


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       white space
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In  PCRE2, POSIX character set names recognize only ASCII characters by
       default, but some of them use Unicode properties if PCRE2_UCP  is  set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy
         {,m}        zero up to m, greedy
         {,m}+       zero up to m, possessive
         {,m}?       zero up to m, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                       also after an internal newline in multiline mode
                       (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
         \A          start of subject
         $           end of subject
                       also before newline at end of subject
                       also before internal newline in multiline mode
         \Z          end of subject
                       also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


REPORTED MATCH POINT SETTING

         \K          set reported start of match

       From  release 10.38 \K is not permitted by default in lookaround asser-
       tions, for compatibility with Perl.  However,  if  the  PCRE2_EXTRA_AL-
       LOW_LOOKAROUND_BSK option is set, the previous behaviour is re-enabled.
       When this option is set, \K is honoured in positive assertions, but ig-
       nored in negative ones.


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capture group
         (?<name>...)    named capture group (Perl)
         (?'name'...)    named capture group (Perl)
         (?P<name>...)   named capture group (Python)
         (?:...)         non-capture group
         (?|...)         non-capture group; reset group numbers for
                          capture groups in each alternative

       In  non-UTF  modes, names may contain underscores and ASCII letters and
       digits; in UTF modes, any Unicode letters and  Unicode  decimal  digits
       are permitted. In both cases, a name must not start with a digit.


ATOMIC GROUPS

         (?>...)         atomic non-capture group
         (*atomic:...)   atomic non-capture group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING
       Changes  of these options within a group are automatically cancelled at
       the end of the group.

         (?a)            all ASCII options
         (?aD)           restrict \d to ASCII in UCP mode
         (?aS)           restrict \s to ASCII in UCP mode
         (?aW)           restrict \w to ASCII in UCP mode
         (?aP)           restrict all POSIX classes to ASCII in UCP mode
         (?aT)           restrict POSIX digit classes to ASCII in UCP mode
         (?i)            caseless
         (?J)            allow duplicate named groups
         (?m)            multiline
         (?n)            no auto capture
         (?r)            restrict caseless to either ASCII or non-ASCII
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            ignore white space except in classes or \Q...\E
         (?xx)           as (?x) but also ignore space and tab in classes
         (?-...)         unset the given option(s)
         (?^)            unset imnrsx options

       (?aP) implies (?aT) as well, though this has no additional effect. How-
       ever, it means that (?-aP) is really (?-PT) which  disables  all  ASCII
       restrictions for POSIX classes.

       Unsetting  x or xx unsets both. Several options may be set at once, and
       a mixture of setting and unsetting such as (?i-x) is allowed, but there
       may be only one hyphen. Setting (but no unsetting) is allowed after (?^
       for example (?^in). An option setting may appear at the start of a non-
       capture group, for example (?i:...).

       The following are recognized only at the very start of a pattern or af-
       ter one of the newline or \R options with similar syntax. More than one
       of them may appear. For the first three, d is a decimal number.

         (*LIMIT_DEPTH=d) set the backtracking limit to d
         (*LIMIT_HEAP=d)  set the heap size limit to d * 1024 bytes
         (*LIMIT_MATCH=d) set the match limit to d
         (*NOTEMPTY)      set PCRE2_NOTEMPTY when matching
         (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching
         (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS)
         (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
         (*NO_JIT)       disable JIT optimization
         (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE)
         (*UTF)          set appropriate UTF mode for the library in use
         (*UCP)          set PCRE2_UCP (use Unicode properties for \d etc)

       Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce  the
       value   of   the   limits   set  by  the  caller  of  pcre2_match()  or
       pcre2_dfa_match(), not increase them. LIMIT_RECURSION  is  an  obsolete
       synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF)
       and  (*UCP)  by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options,
       respectively, at compile time.


NEWLINE CONVENTION

       These are recognized only at the very start of the pattern or after op-
       tion settings with a similar syntax.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence
         (*NUL)          the NUL character (binary zero)


WHAT \R MATCHES

       These are recognized only at the very start of the pattern or after op-
       tion setting with a similar syntax.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)                     )
         (*pla:...)                  ) positive lookahead
         (*positive_lookahead:...)   )

         (?!...)                     )
         (*nla:...)                  ) negative lookahead
         (*negative_lookahead:...)   )

         (?<=...)                    )
         (*plb:...)                  ) positive lookbehind
         (*positive_lookbehind:...)  )

         (?<!...)                    )
         (*nlb:...)                  ) negative lookbehind
         (*negative_lookbehind:...)  )

       Each top-level branch of a lookbehind must have a limit for the  number
       of  characters it matches. If any branch can match a variable number of
       characters, the maximum for each branch is limited to a  value  set  by
       the  caller  of  pcre2_compile()  or defaulted. The default is set when
       PCRE2 is built (ultimate default 255). If every branch matches a  fixed
       number of characters, the limit for each branch is 65535 characters.


NON-ATOMIC LOOKAROUND ASSERTIONS

       These assertions are specific to PCRE2 and are not Perl-compatible.

         (?*...)                                )
         (*napla:...)                           ) synonyms
         (*non_atomic_positive_lookahead:...)   )

         (?<*...)                               )
         (*naplb:...)                           ) synonyms
         (*non_atomic_positive_lookbehind:...)  )


SCRIPT RUNS

         (*script_run:...)           ) script run, can be backtracked into
         (*sr:...)                   )

         (*atomic_script_run:...)    ) atomic script run
         (*asr:...)                  )


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g+n            relative reference by number (PCRE2 extension)
         \g-n            relative reference by number
         \g{+n}          relative reference by number (PCRE2 extension)
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subroutine by absolute number
         (?+n)           call subroutine by relative number
         (?-n)           call subroutine by relative number
         (?&name)        call subroutine by name (Perl)
         (?P>name)       call subroutine by name (Python)
         \g<name>        call subroutine by name (Oniguruma)
         \g'name'        call subroutine by name (Oniguruma)
         \g<n>           call subroutine by absolute number (Oniguruma)
         \g'n'           call subroutine by absolute number (Oniguruma)
         \g<+n>          call subroutine by relative number (PCRE2 extension)
         \g'+n'          call subroutine by relative number (PCRE2 extension)
         \g<-n>          call subroutine by relative number (PCRE2 extension)
         \g'-n'          call subroutine by relative number (PCRE2 extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)               absolute reference condition
         (?(+n)              relative reference condition (PCRE2 extension)
         (?(-n)              relative reference condition (PCRE2 extension)
         (?(<name>)          named reference condition (Perl)
         (?('name')          named reference condition (Perl)
         (?(name)            named reference condition (PCRE2, deprecated)
         (?(R)               overall recursion condition
         (?(Rn)              specific numbered group recursion condition
         (?(R&name)          specific named group recursion condition
         (?(DEFINE)          define groups for reference
         (?(VERSION[>]=n.m)  test PCRE2 version
         (?(assert)          assertion condition

       Note  the  ambiguity of (?(R) and (?(Rn) which might be named reference
       conditions or recursion tests. Such a condition  is  interpreted  as  a
       reference condition if the relevant named group exists.


BACKTRACKING CONTROL

       All  backtracking  control  verbs  may be in the form (*VERB:NAME). For
       (*MARK) the name is mandatory, for the others it is  optional.  (*SKIP)
       changes  its  behaviour if :NAME is present. The others just set a name
       for passing back to the caller, but this is not a name that (*SKIP) can
       see. The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The following act only when a subsequent match failure causes  a  back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation

       The  effect  of one of these verbs in a group called as a subroutine is
       confined to the subroutine call.


CALLOUTS

         (?C)            callout (assumed number 0)
         (?Cn)           callout with numerical data n
         (?C"text")      callout with string data

       The allowed string delimiters are ` ' " ^ % # $ (which are the same for
       the start and the end), and the starting delimiter { matched  with  the
       ending  delimiter  }. To encode the ending delimiter within the string,
       double it.


SEE ALSO

       pcre2pattern(3),   pcre2api(3),   pcre2callout(3),    pcre2matching(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 12 October 2023
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.43                     12 October 2023                 PCRE2SYNTAX(3)
------------------------------------------------------------------------------



PCRE2UNICODE(3)            Library Functions Manual            PCRE2UNICODE(3)


NAME
       PCRE - Perl-compatible regular expressions (revised API)


UNICODE AND UTF SUPPORT

       PCRE2 is normally built with Unicode support, though if you do not need
       it,  you  can  build  it  without,  in  which  case the library will be
       smaller. With Unicode support, PCRE2 has knowledge of Unicode character
       properties and can process strings of text in UTF-8, UTF-16, and UTF-32
       format (depending on the code unit width), but this is not the default.
       Unless specifically requested, PCRE2 treats each code unit in a  string
       as one character.

       There  are two ways of telling PCRE2 to switch to UTF mode, where char-
       acters may consist of more than one code unit and the range  of  values
       is constrained. The program can call pcre2_compile() with the PCRE2_UTF
       option,  or  the  pattern may start with the sequence (*UTF).  However,
       the latter facility can be locked out by  the  PCRE2_NEVER_UTF  option.
       That  is,  the  programmer can prevent the supplier of the pattern from
       switching to UTF mode.

       Note  that  the  PCRE2_MATCH_INVALID_UTF  option  (see  below)   forces
       PCRE2_UTF to be set.

       In  UTF mode, both the pattern and any subject strings that are matched
       against it are treated as UTF strings instead of strings of  individual
       one-code-unit  characters. There are also some other changes to the way
       characters are handled, as documented below.


UNICODE PROPERTY SUPPORT

       When PCRE2 is built with Unicode support, the escape sequences  \p{..},
       \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF set-
       ting.   The Unicode properties that can be tested are a subset of those
       that Perl supports. Currently they are limited to the general  category
       properties such as Lu for an upper case letter or Nd for a decimal num-
       ber, the derived properties Any and LC (synonym L&), the Unicode script
       names such as Arabic or Han, Bidi_Class, Bidi_Control, and a few binary
       properties.

       The full lists are given in the pcre2pattern and pcre2syntax documenta-
       tion.  In  general,  only the short names for properties are supported.
       For example, \p{L} matches a letter. Its longer synonym, \p{Letter}, is
       not supported. Furthermore, in Perl, many properties may optionally  be
       prefixed  by "Is", for compatibility with Perl 5.6. PCRE2 does not sup-
       port this.


WIDE CHARACTERS AND UTF MODES

       Code points less than 256 can be specified in patterns by either braced
       or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
       Larger values have to use braced sequences. Unbraced octal code  points
       up to \777 are also recognized; larger ones can be coded using \o{...}.

       The  escape sequence \N{U+<hex digits>} is recognized as another way of
       specifying a Unicode character by code point in a UTF mode. It  is  not
       allowed in non-UTF mode.

       In  UTF  mode, repeat quantifiers apply to complete UTF characters, not
       to individual code units.

       In UTF mode, the dot metacharacter matches one UTF character instead of
       a single code unit.

       In UTF mode, capture group names are not restricted to ASCII,  and  may
       contain any Unicode letters and decimal digits, as well as underscore.

       The  escape  sequence \C can be used to match a single code unit in UTF
       mode, but its use can lead to some strange effects because it breaks up
       multi-unit characters (see the description of \C  in  the  pcre2pattern
       documentation). For this reason, there is a build-time option that dis-
       ables  support  for  \C completely. There is also a less draconian com-
       pile-time option for locking out the use of \C when a pattern  is  com-
       piled.

       The  use  of  \C  is not supported by the alternative matching function
       pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac-
       ter may consist of more than one code unit. The  use  of  \C  in  these
       modes  provokes a match-time error. Also, the JIT optimization does not
       support \C in these modes. If JIT optimization is requested for a UTF-8
       or UTF-16 pattern that contains \C, it will not succeed,  and  so  when
       pcre2_match() is called, the matching will be carried out by the inter-
       pretive function.

       The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
       characters  of  any  code  value,  but, by default, the characters that
       PCRE2 recognizes as digits, spaces, or word characters remain the  same
       set  as  in  non-UTF mode, all with code points less than 256. This re-
       mains true even when PCRE2 is built to include Unicode support, because
       to do otherwise would slow down matching in  many  common  cases.  Note
       that  this also applies to \b and \B, because they are defined in terms
       of \w and \W. If you want to test for a wider sense of,  say,  "digit",
       you  can  use  explicit Unicode property tests such as \p{Nd}. Alterna-
       tively, if you set the PCRE2_UCP option, the way that the character es-
       capes work is changed so that Unicode properties are used to  determine
       which  characters  match,  though  there are some options that suppress
       this for individual escapes. For details see  the  section  on  generic
       character types in the pcre2pattern documentation.

       Like  the  escapes,  characters  that  match  the POSIX named character
       classes are all low-valued characters unless the  PCRE2_UCP  option  is
       set, but there is an option to override this.

       In contrast to the character escapes and character classes, the special
       horizontal  and  vertical  white  space escapes (\h, \H, \v, and \V) do
       match all the appropriate Unicode characters, whether or not  PCRE2_UCP
       is set.


UNICODE CASE-EQUIVALENCE

       If  either  PCRE2_UTF  or PCRE2_UCP is set, upper/lower case processing
       makes use of Unicode properties except for characters whose code points
       are less than 128 and that have at most two case-equivalent values. For
       these, a direct table lookup is used for speed. A few  Unicode  charac-
       ters  such as Greek sigma have more than two code points that are case-
       equivalent, and these are treated specially. Setting PCRE2_UCP  without
       PCRE2_UTF  allows  Unicode-style  case processing for non-UTF character
       encodings such as UCS-2.

       There are two ASCII characters (S and K) that,  in  addition  to  their
       ASCII  lower case equivalents, have a non-ASCII one as well (long S and
       Kelvin sign).  Recognition of these non-ASCII characters as case-equiv-
       alent to their ASCII  counterparts  can  be  disabled  by  setting  the
       PCRE2_EXTRA_CASELESS_RESTRICT  option. When this is set, all characters
       in a case equivalence must either be ASCII or non-ASCII; there  can  be
       no mixing.


SCRIPT RUNS

       The  pattern constructs (*script_run:...) and (*atomic_script_run:...),
       with synonyms (*sr:...) and (*asr:...), verify that the string  matched
       within  the  parentheses is a script run. In concept, a script run is a
       sequence of characters that are all from the same Unicode script.  How-
       ever, because some scripts are commonly used together, and because some
       diacritical  and  other marks are used with multiple scripts, it is not
       that simple.

       Every Unicode character has a Script property, mostly with a value cor-
       responding to the name of a script, such as Latin, Greek, or  Cyrillic.
       There are also three special values:

       "Unknown" is used for code points that have not been assigned, and also
       for  the surrogate code points. In the PCRE2 32-bit library, characters
       whose code points are greater  than  the  Unicode  maximum  (U+10FFFF),
       which  are  accessible  only  in non-UTF mode, are assigned the Unknown
       script.

       "Common" is used for characters that are used with many scripts.  These
       include  punctuation,  emoji,  mathematical, musical, and currency sym-
       bols, and the ASCII digits 0 to 9.

       "Inherited" is used for characters such as diacritical marks that  mod-
       ify a previous character. These are considered to take on the script of
       the character that they modify.

       Some  Inherited characters are used with many scripts, but many of them
       are only normally used with a small number  of  scripts.  For  example,
       U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop-
       tic.  In  order  to  make it possible to check this, a Unicode property
       called Script Extension exists. Its value is a list of scripts that ap-
       ply to the character. For the majority of characters, the list contains
       just one script, the same one as  the  Script  property.  However,  for
       characters  such  as  U+102E0 more than one Script is listed. There are
       also some Common characters that have a single,  non-Common  script  in
       their Script Extension list.

       The next section describes the basic rules for deciding whether a given
       string  of  characters  is  a script run. Note, however, that there are
       some special cases involving the Chinese Han script, and an  additional
       constraint  for  decimal  digits.  These are covered in subsequent sec-
       tions.

   Basic script run rules

       A string that is less than two characters long is a script run. This is
       the only case in which an Unknown character can be  part  of  a  script
       run.  Longer strings are checked using only the Script Extensions prop-
       erty, not the basic Script property.

       If a character's Script Extension property is the single value  "Inher-
       ited", it is always accepted as part of a script run. This is also true
       for  the  property  "Common", subject to the checking of decimal digits
       described below. All the remaining characters in a script run must have
       at least one script in common in their Script Extension lists. In  set-
       theoretic terminology, the intersection of all the sets of scripts must
       not be empty.

       A  simple example is an Internet name such as "google.com". The letters
       are all in the Latin script, and the dot is Common, so this string is a
       script run.  However, the Cyrillic letter "o" looks exactly the same as
       the Latin "o"; a string that looks the same, but with Cyrillic "o"s  is
       not a script run.

       More  interesting examples involve characters with more than one script
       in their Script Extension. Consider the following characters:

         U+060C  Arabic comma
         U+06D4  Arabic full stop

       The first has the Script Extension list Arabic, Hanifi  Rohingya,  Syr-
       iac,  and  Thaana; the second has just Arabic and Hanifi Rohingya. Both
       of them could appear in script runs of  either  Arabic  or  Hanifi  Ro-
       hingya.  The  first  could also appear in Syriac or Thaana script runs,
       but the second could not.

   The Chinese Han script

       The Chinese Han script is  commonly  used  in  conjunction  with  other
       scripts  for  writing certain languages. Japanese uses the Hiragana and
       Katakana scripts together with Han; Korean uses Hangul  and  Han;  Tai-
       wanese  Mandarin  uses  Bopomofo  and Han. These three combinations are
       treated as special cases when checking script runs and are, in  effect,
       "virtual  scripts".  Thus,  a script run may contain a mixture of Hira-
       gana, Katakana, and Han, or a mixture of Hangul and Han, or  a  mixture
       of  Bopomofo  and  Han,  but  not, for example, a mixture of Hangul and
       Bopomofo and Han. PCRE2 (like Perl) follows Unicode's  Technical  Stan-
       dard   39   ("Unicode   Security   Mechanisms",  http://unicode.org/re-
       ports/tr39/) in allowing such mixtures.

   Decimal digits

       Unicode contains many sets of 10 decimal digits in  different  scripts,
       and  some  scripts  (including the Common script) contain more than one
       set. Some of these decimal digits them are  visually  indistinguishable
       from  the  common  ASCII digits. In addition to the script checking de-
       scribed above, if a script run contains any decimal digits,  they  must
       all come from the same set of 10 adjacent characters.


VALIDITY OF UTF STRINGS

       When  the  PCRE2_UTF  option is set, the strings passed as patterns and
       subjects are (by default) checked for validity on entry to the relevant
       functions. If an invalid UTF string is passed, a negative error code is
       returned. The code unit offset to the offending character  can  be  ex-
       tracted  from  the  match  data block by calling pcre2_get_startchar(),
       which is used for this purpose after a UTF error.

       In some situations, you may already know that your strings  are  valid,
       and  therefore  want  to  skip these checks in order to improve perfor-
       mance, for example in the case of a long subject string that  is  being
       scanned  repeatedly.   If you set the PCRE2_NO_UTF_CHECK option at com-
       pile time or at match time, PCRE2 assumes that the pattern  or  subject
       it is given (respectively) contains only valid UTF code unit sequences.

       If  you  pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the
       result is undefined and your program may crash or loop indefinitely  or
       give  incorrect  results.  There is, however, one mode of matching that
       can handle invalid UTF subject strings.  This  is  enabled  by  passing
       PCRE2_MATCH_INVALID_UTF  to  pcre2_compile()  and is discussed below in
       the next section. The  rest  of  this  section  covers  the  case  when
       PCRE2_MATCH_INVALID_UTF is not set.

       Passing  PCRE2_NO_UTF_CHECK  to  pcre2_compile()  just disables the UTF
       check for the pattern; it does not also apply to  subject  strings.  If
       you  want  to disable the check for a subject string you must pass this
       same option to pcre2_match() or pcre2_dfa_match().

       UTF-16 and UTF-32 strings can indicate their endianness by special code
       knows as a byte-order mark (BOM). The PCRE2  functions  do  not  handle
       this, expecting strings to be in host byte order.

       Unless  PCRE2_NO_UTF_CHECK  is  set, a UTF string is checked before any
       other  processing  takes  place.  In  the  case  of  pcre2_match()  and
       pcre2_dfa_match()  calls  with a non-zero starting offset, the check is
       applied only to that part of the subject that could be inspected during
       matching, and there is a check that the starting offset points  to  the
       first  code  unit of a character or to the end of the subject. If there
       are no lookbehind assertions in the pattern, the check  starts  at  the
       starting  offset.   Otherwise,  it  starts at the length of the longest
       lookbehind before the starting offset, or at the start of  the  subject
       if  there are not that many characters before the starting offset. Note
       that the sequences \b and \B are one-character lookbehinds.

       In addition to checking the format of the string, there is a  check  to
       ensure that all code points lie in the range U+0 to U+10FFFF, excluding
       the  surrogate  area. The so-called "non-character" code points are not
       excluded because Unicode corrigendum #9 makes it clear that they should
       not be.

       Characters in the "Surrogate Area" of Unicode are reserved for  use  by
       UTF-16,  where they are used in pairs to encode code points with values
       greater than 0xFFFF. The code points that are encoded by  UTF-16  pairs
       are  available  independently  in  the  UTF-8 and UTF-32 encodings. (In
       other words, the whole surrogate thing is a fudge for UTF-16 which  un-
       fortunately messes up UTF-8 and UTF-32.)

       Setting  PCRE2_NO_UTF_CHECK  at compile time does not disable the error
       that is given if an escape sequence for an invalid Unicode  code  point
       is  encountered  in  the pattern. If you want to allow escape sequences
       such as \x{d800} (a surrogate code point) you  can  set  the  PCRE2_EX-
       TRA_ALLOW_SURROGATE_ESCAPES  extra  option.  However,  this is possible
       only in UTF-8 and UTF-32 modes, because these  values  are  not  repre-
       sentable in UTF-16.

   Errors in UTF-8 strings

       The following negative error codes are given for invalid UTF-8 strings:

         PCRE2_ERROR_UTF8_ERR1
         PCRE2_ERROR_UTF8_ERR2
         PCRE2_ERROR_UTF8_ERR3
         PCRE2_ERROR_UTF8_ERR4
         PCRE2_ERROR_UTF8_ERR5

       The  string  ends  with a truncated UTF-8 character; the code specifies
       how many bytes are missing (1 to 5). Although RFC 3629 restricts  UTF-8
       characters  to  be  no longer than 4 bytes, the encoding scheme (origi-
       nally defined by RFC 2279) allows for  up  to  6  bytes,  and  this  is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE2_ERROR_UTF8_ERR6
         PCRE2_ERROR_UTF8_ERR7
         PCRE2_ERROR_UTF8_ERR8
         PCRE2_ERROR_UTF8_ERR9
         PCRE2_ERROR_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the  character  do  not have the binary value 0b10 (that is, either the
       most significant bit is 0, or the next bit is 1).

         PCRE2_ERROR_UTF8_ERR11
         PCRE2_ERROR_UTF8_ERR12

       A character that is valid by the RFC 2279 rules is either 5 or 6  bytes
       long; these code points are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR13

       A 4-byte character has a value greater than 0x10ffff; these code points
       are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR14

       A  3-byte  character  has  a  value in the range 0xd800 to 0xdfff; this
       range of code points are reserved by RFC 3629 for use with UTF-16,  and
       so are excluded from UTF-8.

         PCRE2_ERROR_UTF8_ERR15
         PCRE2_ERROR_UTF8_ERR16
         PCRE2_ERROR_UTF8_ERR17
         PCRE2_ERROR_UTF8_ERR18
         PCRE2_ERROR_UTF8_ERR19

       A  2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
       for a value that can be represented by fewer bytes, which  is  invalid.
       For  example,  the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
       rect coding uses just one byte.

         PCRE2_ERROR_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary value 0b10 (that is, the most significant bit is 1 and the  sec-
       ond  is  0). Such a byte can only validly occur as the second or subse-
       quent byte of a multi-byte character.

         PCRE2_ERROR_UTF8_ERR21

       The first byte of a character has the value 0xfe or 0xff. These  values
       can never occur in a valid UTF-8 string.

   Errors in UTF-16 strings

       The  following  negative  error  codes  are  given  for  invalid UTF-16
       strings:

         PCRE2_ERROR_UTF16_ERR1  Missing low surrogate at end of string
         PCRE2_ERROR_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE2_ERROR_UTF16_ERR3  Isolated low surrogate


   Errors in UTF-32 strings

       The following  negative  error  codes  are  given  for  invalid  UTF-32
       strings:

         PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
         PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff


MATCHING IN INVALID UTF STRINGS

       You can run pattern matches on subject strings that may contain invalid
       UTF  sequences  if  you  call  pcre2_compile() with the PCRE2_MATCH_IN-
       VALID_UTF option. This is supported  by  pcre2_match(),  including  JIT
       matching, but not by pcre2_dfa_match(). When PCRE2_MATCH_INVALID_UTF is
       set,  it  forces  PCRE2_UTF  to be set as well. Note, however, that the
       pattern itself must be a valid UTF string.

       If you do not set PCRE2_MATCH_INVALID_UTF when  calling  pcre2_compile,
       and  you  are  not  certain that your subject strings are valid UTF se-
       quences, you should not make  use  of  the  JIT  "fast  path"  function
       pcre2_jit_match()  because it bypasses sanity checks, including the one
       for UTF validity. An invalid string may cause undefined behaviour,  in-
       cluding looping, crashing, or giving the wrong answer.

       Setting  PCRE2_MATCH_INVALID_UTF  does  not affect what pcre2_compile()
       generates, but if pcre2_jit_compile() is subsequently called,  it  does
       generate different code. If JIT is not used, the option affects the be-
       haviour of the interpretive code in pcre2_match(). When PCRE2_MATCH_IN-
       VALID_UTF  is  set  at  compile  time, PCRE2_NO_UTF_CHECK is ignored at
       match time.

       In this mode, an invalid  code  unit  sequence  in  the  subject  never
       matches  any  pattern  item.  It  does not match dot, it does not match
       \p{Any}, it does not even match negative items such as [^X]. A  lookbe-
       hind  assertion fails if it encounters an invalid sequence while moving
       the current point backwards. In other words, an invalid UTF  code  unit
       sequence acts as a barrier which no match can cross.

       You can also think of this as the subject being split up into fragments
       of  valid UTF, delimited internally by invalid code unit sequences. The
       pattern is matched fragment by fragment. The  result  of  a  successful
       match,  however,  is  given  as code unit offsets in the entire subject
       string in the usual way. There are a few points to consider:

       The internal boundaries are not interpreted as the beginnings  or  ends
       of  lines  and  so  do not match circumflex or dollar characters in the
       pattern.

       If pcre2_match() is called with an offset that  points  to  an  invalid
       UTF-sequence,  that  sequence  is  skipped, and the match starts at the
       next valid UTF character, or the end of the subject.

       At internal fragment boundaries, \b and \B behave in the same way as at
       the beginning and end of the subject. For example, a sequence  such  as
       \bWORD\b  would match an instance of WORD that is surrounded by invalid
       UTF code units.

       Using PCRE2_MATCH_INVALID_UTF, an application can run matches on  arbi-
       trary  data,  knowing  that  any  matched strings that are returned are
       valid UTF. This can be useful when searching for UTF text in executable
       or other binary files.

       Note, however, that the  16-bit  and  32-bit  PCRE2  libraries  process
       strings  as  sequences of uint16_t or uint32_t code points. They cannot
       find valid UTF sequences within an arbitrary  string  of  bytes  unless
       such sequences are suitably aligned.


AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


REVISION

       Last updated: 12 October 2023
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.43                    04 February 2023                PCRE2UNICODE(3)
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