PCRE(3)PCRE(3)NAMEpcreposix - POSIX API for Perl-compatible regular expres
sions.
SYNOPSIS
#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 pcre documenta
tion for a description of the native API, which contains
additional functionality.
The functions described here are just wrapper functions
that ultimately call the native API. Their prototypes are
defined in the pcreposix.h header file, and on Unix sys
tems the library itself is called pcreposix.a, so can be
accessed by adding -lpcreposix to the command for linking
an application which uses them. Because the POSIX func
tions call the native ones, it is also necessary to add
-lpcre.
I have implemented only those option bits that can be rea
sonably 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.
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 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 which 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_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.
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_MULTI
LINE has only some of the effects specified for REG_NEW
LINE. It does not affect the way newlines are matched by .
(they aren't) or 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 publicized: re_nsub con
tains the number of capturing subpatterns in the regular
expression. Various error codes are defined in the header
file.
MATCHING A PATTERN
The function regexec() is called to match a pre-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.
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 reg_
match_t, containing the members rm_so and rm_eo. These
contain the offset to the first character of each sub
string 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 sub
patterns 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 function
is the size of buffer needed to hold the whole message.
STORAGE
Compiling a regular expression causes memory to be allo
cated 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.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
New Museums Site,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Copyright (c) 1997-2000 University of Cambridge.
PCRE(3)
