PCRE2POSIX(3)              Library Functions Manual              PCRE2POSIX(3)

       PCRE2 - Perl-compatible regular expressions (revised API)


       #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);


       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 func-

       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_. How-
       ever, the pcre2posix.h header also contains macro definitions that con-
       vert  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

       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 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.


       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.


       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:


       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.


       The  PCRE2_CASELESS option is set when the regular expression is passed
       for compilation to the native function.


       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-


       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.


       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 back-


       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  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.


       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.


       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.


       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 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
       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.


       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


       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:


       The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
       ing function.


       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.


       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
       ing function.


       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

       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

       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-

       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.


       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  buf-
       fer  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.


       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


       Philip Hazel
       University Computing Service
       Cambridge, England.


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

PCRE2 10.37                      26 April 2021                   PCRE2POSIX(3)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2022 Hurricane Electric. All Rights Reserved.