.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 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

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:

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  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).

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  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_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 aren't) 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.

.

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.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:

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                          Default   Change with

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  . 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

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This is the equivalent table for POSIX:

.sp

                          Default   Change with

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  . 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.

.

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.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:

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  REG_NOTBOL

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The PCRE_NOTBOL option is set when calling the underlying PCRE matching

function.

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  REG_NOTEOL

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The PCRE_NOTEOL option is set when calling the underlying PCRE matching

function.

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  REG_STARTEND

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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.

.

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.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.

.

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.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.

.

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.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: 05 April 2008

Copyright (c) 1997-2008 University of Cambridge.

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