Manual Reference Pages - PCREPOSIX (3)
PCRE - Perl-compatible regular expressions.
int regcomp(regex_t *preg, const char *pattern,
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);
This set of functions provides a POSIX-style API for the PCRE regular
expression 8-bit library. See the
documentation for a description of PCREs native API, which contains much
additional functionality. There is no POSIX-style wrapper for PCREs 16-bit
and 32-bit library.
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 POSIX 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.
There are also some other options that are not defined by POSIX. These have
been added at the request of users who want to make use of certain
PCRE-specific features via the POSIX calling interface.
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.
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:
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
The PCRE_CASELESS option is set when the regular expression is passed for
compilation to the native function.
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 section).
The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed
for compilation to the native function. In addition, when a pattern that is
compiled with this flag is passed to regexec() for matching, the
nmatch and pmatch arguments are ignored, and no captured strings
The PCRE_UCP option is set when the regular expression is passed for
compilation to the native function. This causes PCRE to use Unicode properties
when matchine \d, \w, etc., instead of just recognizing ASCII values. Note
that REG_UTF8 is not part of the POSIX standard.
The PCRE_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
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 are not) or by a negative class such as [^a]
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.
NOTE: If the yield of regcomp() is non-zero, you must not attempt to
use the contents of the preg structure. If, for example, you pass it to
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 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
PCREs behaviour is the same as Perls, 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
MATCHING A PATTERN
The function 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
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
The PCRE_NOTEMPTY option is set when calling the underlying PCRE 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.
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
The string is considered to start at string + pmatch.rm_so and
to have a terminating NUL located at string + pmatch.rm_eo
(there need not actually be a NUL at that location), regardless of the value of
nmatch. 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 of the string, not
how it is matched.
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.
If the value of nmatch is zero, or if the value pmatch is NULL,
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 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 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.
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.
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 09 January 2012
Copyright (c) 1997-2012 University of Cambridge.
|PCRE 8.30 ||PCREPOSIX (3) ||09 January 2012 |
Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.