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NAME

       PCRE - Perl-compatible regular expressions.


SYNOPSIS OF POSIX API


       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);


DESCRIPTION


       This  set  of  functions provides a POSIX-style API to the PCRE regular
       expression package. See the pcreapi documentation for a description  of
       PCRE's native API, which contains much additional functionality.

       The functions described here are just wrapper functions that ultimately
       call  the  PCRE  native  API.  Their  prototypes  are  defined  in  the
       pcreposix.h  header  file,  and  on  Unix systems the library itself is
       called pcreposix.a, so can be accessed by  adding  -lpcreposix  to  the
       command  for  linking  an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I have implemented only those option bits that can be reasonably mapped
       to PCRE native options. 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 PCRE as a replacement library. Other POSIX options are not even
       defined.

       When  PCRE  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 PCRE 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-byte encoding  domains  it  is  probably
       even less compatible.

       The  header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX  libraries.  It  can,  of  course,  be
       renamed or aliased as regex.h, which is the "correct" name. It provides
       two structure types, regex_t for  compiled  internal  forms,  and  reg-
       match_t  for  returning  captured substrings. It also defines some con-
       stants whose names start  with  "REG_";  these  are  used  for  setting
       options and identifying error codes.


COMPILING A PATTERN


       The  function regcomp() is called to compile a pattern into an internal
       form. The pattern is a C string terminated by a  binary  zero,  and  is
       passed  in  the  argument  pattern. 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.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

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

         REG_NEWLINE

       The  PCRE_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_NOSUB

       The PCRE_NO_AUTO_CAPTURE option is set when the regular  expression  is
       passed for compilation to the native function. In addition, when a pat-
       tern that is compiled with this flag is passed to regexec() for  match-
       ing,  the  nmatch  and  pmatch  arguments  are ignored, and no captured
       strings are returned.

         REG_UTF8

       The PCRE_UTF8 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_UTF8 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  PCRE  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
       PCRE_MULTILINE  has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they  aren't)  or
       by a negative class such as [^a] (they are).

       The  yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       is  public: re_nsub contains the number of capturing subpatterns in the
       regular expression. Various error codes are defined in the header file.


MATCHING NEWLINE CHARACTERS


       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE to obey  POSIX  semantics,  but
       then  PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 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

       PCRE's behaviour is the same as Perl's, except that there is no equiva-
       lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,  there  is
       no way to stop newline from matching [^a].

       The   default  POSIX  newline  handling  can  be  obtained  by  setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to  make  PCRE
       behave exactly as for the REG_NEWLINE action.


MATCHING A PATTERN


       The  function  regexec()  is  called  to  match a compiled pattern preg
       against a given string, which is terminated by a zero byte, subject  to
       the options in eflags. These can be:

         REG_NOTBOL

       The PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEOL

       The PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings  is  returned.  The  nmatch  and  pmatch  arguments  of
       regexec() are ignored.

       Otherwise,the portion of the string that was matched, and also any cap-
       tured substrings, are returned via the pmatch argument, which points to
       an  array  of nmatch structures of type regmatch_t, containing the mem-
       bers rm_so and rm_eo. These contain the 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 regerror() function maps a non-zero errorcode from either regcomp()
       or 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.  The  length  of  the  message,
       including  the  zero, is limited to errbuf_size. The yield of the func-
       tion is the size of buffer needed to hold the whole message.


MEMORY USAGE


       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with  the preg structure. The function regfree() frees all such
       memory, after which preg may no longer be used as  a  compiled  expres-
       sion.


AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION


       Last updated: 06 March 2007
       Copyright (c) 1997-2007 University of Cambridge.

                                                                  PCREPOSIX(3)

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