регулярные выражения, совместимые с Perl (Perl-compatible regular expressions.)
Имя (Name)
PCRE - Perl-compatible regular expressions.
Синопсис (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);
Описание (Description)
This set of functions provides a POSIX-style API for the PCRE
regular expression 8-bit library. See the pcreapi documentation
for a description of PCRE's native API, which contains much
additional functionality. There is no POSIX-style wrapper for
PCRE's 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:
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_UCP
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.
REG_UNGREEDY
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 POSIX standard.
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 are not) 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.
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
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.
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 be:
REG_NOTBOL
The PCRE_NOTBOL option is set when calling the underlying PCRE
matching function.
REG_NOTEMPTY
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.
REG_NOTEOL
The PCRE_NOTEOL option is set when calling the underlying PCRE
matching function.
REG_STARTEND
The string is considered to start at string + pmatch[0].rm_so and
to have a terminating NUL located at string + pmatch[0].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.
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.
MEMORY 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.
