pcreposix man page
Return to the 
index.htmlPCRE index page .
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
#SEC1SYNOPSIS OF POSIX API #SEC2DESCRIPTION #SEC3COMPILING A PATTERN #SEC4MATCHING NEWLINE CHARACTERS #SEC5MATCHING A PATTERN #SEC6ERROR MESSAGES #SEC7MEMORY USAGE #SEC8AUTHOR #TOC1SYNOPSIS 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);
#TOC1DESCRIPTION This set of functions provides a POSIX-style API to the PCRE regular expression
package. See the
pcreapi.htmlpcreapi 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.hheader 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 options REG_EXTENDED and REG_NOSUB are defined
with the value zero. They have no effect, but since programs that are written
to the POSIX interface often use them, 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 expressions 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
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.
#TOC1COMPILING 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 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 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 expression is passed for compilation
to the native function.
  REG_NEWLINE
The PCRE_MULTILINE option is set when the 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).
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.
#TOC1MATCHING 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 equivalent 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.
#TOC1MATCHING A PATTERN The function 
regexec() is called to match a compiled pattern pregagainst 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.
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 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.
#TOC1ERROR 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
function is the size of buffer needed to hold the whole message.
#TOC1MEMORY USAGE Compiling a regular expression causes memory to be allocated and associated
with the 
preg structure. The function regfree() frees all such
memory, after which 
preg may no longer be used as a compiled expression.
#TOC1AUTHOR Philip Hazel
University Computing Service,
Cambridge CB2 3QG, England.
Last updated: 28 February 2005
Copyright © 1997-2005 University of Cambridge.
Return to the 
index.htmlPCRE index page .
