path: root/doc/pcreposix.3

.TH PCREPOSIX 3
.SH NAME
PCRE - Perl-compatible regular expressions.
.SH "SYNOPSIS OF POSIX API"
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.B #include <pcreposix.h>
.PP
.SM
.B int regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
.ti +5n
.B int \fIcflags\fP);
.PP
.B int regexec(regex_t *\fIpreg\fP, const char *\fIstring\fP,
.ti +5n
.B size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);
.PP
.B size_t regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,
.ti +5n
.B char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);
.PP
.B void regfree(regex_t *\fIpreg\fP);
.
.SH DESCRIPTION
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This set of functions provides a POSIX-style API to the PCRE regular expression
package. See the
.\" HREF
\fBpcreapi\fP
.\"
documentation for a description of PCRE's native API, which contains much
additional functionality.
.P
The functions described here are just wrapper functions that ultimately call
the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fP
header file, and on Unix systems the library itself is called
\fBpcreposix.a\fP, so can be accessed by adding \fB-lpcreposix\fP to the
command for linking an application that uses them. Because the POSIX functions
call the native ones, it is also necessary to add \fB-lpcre\fP.
.P
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.
.P
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.
.P
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.
.P
The header for these functions is supplied as \fBpcreposix.h\fP to avoid any
potential clash with other POSIX libraries. It can, of course, be renamed or
aliased as \fBregex.h\fP, which is the "correct" name. It provides two
structure types, \fIregex_t\fP for compiled internal forms, and
\fIregmatch_t\fP for returning captured substrings. It also defines some
constants whose names start with "REG_"; these are used for setting options and
identifying error codes.
.P
.SH "COMPILING A PATTERN"
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The function \fBregcomp()\fP 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 \fIpattern\fP. The \fIpreg\fP argument is a pointer
to a \fBregex_t\fP structure that is used as a base for storing information
about the compiled regular expression.
.P
The argument \fIcflags\fP is either zero, or contains one or more of the bits
defined by the following macros:
.sp
  REG_DOTALL
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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.
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  REG_ICASE
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The PCRE_CASELESS option is set when the regular expression is passed for
compilation to the native function.
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  REG_NEWLINE
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The PCRE_MULTILINE option is set when the regular expression is passed for
compilation to the native function. Note that this does \fInot\fP mimic the
defined POSIX behaviour for REG_NEWLINE (see the following section).
.sp
  REG_NOSUB
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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 \fBregexec()\fP for matching, the
\fInmatch\fP and \fIpmatch\fP arguments are ignored, and no captured strings
are returned.
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  REG_UNGREEDY
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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.   
.sp
  REG_UTF8
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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.
.P
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
\fIsome\fP 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).
.P
The yield of \fBregcomp()\fP is zero on success, and non-zero otherwise. The
\fIpreg\fP structure is filled in on success, and one member of the structure
is public: \fIre_nsub\fP contains the number of capturing subpatterns in
the regular expression. Various error codes are defined in the header file.
.P
NOTE: If the yield of \fBregcomp()\fP is non-zero, you must not attempt to
use the contents of the \fIpreg\fP structure. If, for example, you pass it to
\fBregexec()\fP, the result is undefined and your program is likely to crash.
.
.
.SH "MATCHING NEWLINE CHARACTERS"
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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:
.sp
                          Default   Change with
.sp
  . matches newline          no     PCRE_DOTALL
  newline matches [^a]       yes    not changeable
  $ matches \en at end        yes    PCRE_DOLLARENDONLY
  $ matches \en in middle     no     PCRE_MULTILINE
  ^ matches \en in middle     no     PCRE_MULTILINE
.sp
This is the equivalent table for POSIX:
.sp
                          Default   Change with
.sp
  . matches newline          yes    REG_NEWLINE
  newline matches [^a]       yes    REG_NEWLINE
  $ matches \en at end        no     REG_NEWLINE
  $ matches \en in middle     no     REG_NEWLINE
  ^ matches \en in middle     no     REG_NEWLINE
.sp
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].
.P
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.
.
.
.SH "MATCHING A PATTERN"
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The function \fBregexec()\fP is called to match a compiled pattern \fIpreg\fP
against a given \fIstring\fP, which is by default terminated by a zero byte
(but see REG_STARTEND below), subject to the options in \fIeflags\fP. These can
be:
.sp
  REG_NOTBOL
.sp
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
function.
.sp
  REG_NOTEMPTY
.sp
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.
.sp
  REG_NOTEOL
.sp
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
function.
.sp
  REG_STARTEND
.sp
The string is considered to start at \fIstring\fP + \fIpmatch[0].rm_so\fP and
to have a terminating NUL located at \fIstring\fP + \fIpmatch[0].rm_eo\fP
(there need not actually be a NUL at that location), regardless of the value of
\fInmatch\fP. 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 \fIrm_so\fP does
not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not
how it is matched.
.P
If the pattern was compiled with the REG_NOSUB flag, no data about any matched
strings is returned. The \fInmatch\fP and \fIpmatch\fP arguments of
\fBregexec()\fP are ignored.
.P
Otherwise,the portion of the string that was matched, and also any captured
substrings, are returned via the \fIpmatch\fP argument, which points to an
array of \fInmatch\fP structures of type \fIregmatch_t\fP, containing the
members \fIrm_so\fP and \fIrm_eo\fP. 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 \fIstring\fP 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.
.P
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.
.
.
.SH "ERROR MESSAGES"
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The \fBregerror()\fP function maps a non-zero errorcode from either
\fBregcomp()\fP or \fBregexec()\fP to a printable message. If \fIpreg\fP is not
NULL, the error should have arisen from the use of that structure. A message
terminated by a binary zero is placed in \fIerrbuf\fP. The length of the
message, including the zero, is limited to \fIerrbuf_size\fP. The yield of the
function is the size of buffer needed to hold the whole message.
.
.
.SH MEMORY USAGE
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Compiling a regular expression causes memory to be allocated and associated
with the \fIpreg\fP structure. The function \fBregfree()\fP frees all such
memory, after which \fIpreg\fP may no longer be used as a compiled expression.
.
.
.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
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.SH REVISION
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Last updated: 02 September 2009
Copyright (c) 1997-2009 University of Cambridge.
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