PCRE — Perl-compatible regular expressions.
#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) ; |
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 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.
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
section).
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 pattern that is
compiled with this flag is passed to regexec()
for matching,
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.
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.
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 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.
Last updated: 06 March 2007 Copyright (c) 1997-2007 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |