Provided by: libpcre2-dev_10.42-4ubuntu2_amd64 bug

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 description of PCRE2's native API, which contains much
       additional functionality.

       The functions described here 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 library.

       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.

       Although they were not defined as protypes 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  functions.  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 identifying error codes.

USING THE POSIX FUNCTIONS


       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 expressions 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  internal  form.  By
       default,  the  pattern is a C string terminated by a binary 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 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 section).

         REG_NOSPEC

       The  PCRE2_LITERAL  option is set when the regular expression is passed for compilation to
       the native function. This disables all meta characters 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  ignored,  and  no  captured  strings  are  returned.
       Versions  of the PCRE library prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile
       option, but this no longer happens because it disables the use of backreferences.

         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 itself may now contain binary zeros,  which  are  treated  as
       data  characters.  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 matchine \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
       the the regex is compiled with PCRE2 default semantics. 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 metacharacter (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  otherwise.   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 error 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 action. 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 matching 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 behaviour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 matching 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 zeros 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
       implementations.)

       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_STARTEND).

       The value of nmatch may be zero, and the value pmatch may be NULL (unless 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.

       A successful match yields a zero return; various error codes are  defined  in  the  header
       file, of which REG_NOMATCH is the "expected" failure 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 associated with the preg
       structure. The function pcre2_regfree() frees all such memory, after  which  preg  may  no
       longer be used as a compiled expression.

AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION


       Last updated: 26 April 2021
       Copyright (c) 1997-2021 University of Cambridge.