trunk/doc/pcreposix.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions.
.SH SYNOPSIS OF POSIX API
.B #include <pcreposix.h>
.PP
.SM
.br
.B int regcomp(regex_t *\fIpreg\fR, const char *\fIpattern\fR,
.ti +5n
.B int \fIcflags\fR);
.PP
.br
.B int regexec(regex_t *\fIpreg\fR, const char *\fIstring\fR,
.ti +5n
.B size_t \fInmatch\fR, regmatch_t \fIpmatch\fR[], int \fIeflags\fR);
.PP
.br
.B size_t regerror(int \fIerrcode\fR, const regex_t *\fIpreg\fR,
.ti +5n
.B char *\fIerrbuf\fR, size_t \fIerrbuf_size\fR);
.PP
.br
.B void regfree(regex_t *\fIpreg\fR);

.SH DESCRIPTION
.rs
.sp
This set of functions provides a POSIX-style API to the PCRE regular expression
package. See the
.\" HREF
\fBpcreapi\fR
.\"
documentation for a description of the native API, which contains additional
functionality.

The functions described here are just wrapper functions that ultimately call
the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fR
header file, and on Unix systems the library itself is called
\fBpcreposix.a\fR, so can be accessed by adding \fB-lpcreposix\fR to the
command for linking an application which uses them. Because the POSIX functions
call the native ones, it is also necessary to add \fR-lpcre\fR.

I have implemented only those option bits that can be reasonably 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. "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 \fBpcreposix.h\fR to avoid any
potential clash with other POSIX libraries. It can, of course, be renamed or
aliased as \fBregex.h\fR, which is the "correct" name. It provides two
structure types, \fIregex_t\fR for compiled internal forms, and
\fIregmatch_t\fR for returning captured substrings. It also defines some
constants whose names start with "REG_"; these are used for setting options and
identifying error codes.

.SH COMPILING A PATTERN
.rs
.sp
The function \fBregcomp()\fR 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\fR. The \fIpreg\fR argument is a pointer
to a regex_t structure which is used as a base for storing information about
the compiled expression.

The argument \fIcflags\fR 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. Note that this does \fInot\fR mimic the defined POSIX
behaviour for REG_NEWLINE (see the following section).

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\fR 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 \fBregcomp()\fR is zero on success, and non-zero otherwise. The
\fIpreg\fR structure is filled in on success, and one member of the structure
is public: \fIre_nsub\fR contains the number of capturing subpatterns in
the regular expression. Various error codes are defined in the header file.

.SH MATCHING NEWLINE CHARACTERS
.rs
.sp
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_DOLLARENDONLY 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_DOLLARENDONLY, but there is no way to make PCRE behave exactly as for the
REG_NEWLINE action.

.SH MATCHING A PATTERN
.rs
.sp
The function \fBregexec()\fR is called to match a pre-compiled pattern
\fIpreg\fR against a given \fIstring\fR, which is terminated by a zero byte,
subject to the options in \fIeflags\fR. 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 \fIpmatch\fR argument, which points to an array of
\fInmatch\fR structures of type \fIregmatch_t\fR, containing the members
\fIrm_so\fR and \fIrm_eo\fR. 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\fR 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.

.SH ERROR MESSAGES
.rs
.sp
The \fBregerror()\fR function maps a non-zero errorcode from either
\fBregcomp()\fR or \fBregexec()\fR to a printable message. If \fIpreg\fR 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\fR. The length of the
message, including the zero, is limited to \fIerrbuf_size\fR. The yield of the
function is the size of buffer needed to hold the whole message.

.SH STORAGE
.rs
.sp
Compiling a regular expression causes memory to be allocated and associated
with the \fIpreg\fR structure. The function \fBregfree()\fR frees all such
memory, after which \fIpreg\fR may no longer be used as a compiled expression.

.SH AUTHOR
.rs
.sp
Philip Hazel <ph10@cam.ac.uk>
.br
University Computing Service,
.br
Cambridge CB2 3QG, England.

.in 0
Last updated: 03 February 2003
.br
Copyright (c) 1997-2003 University of Cambridge.
1	.TH PCRE 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions.
4	.SH SYNOPSIS OF POSIX API
5	.B #include <pcreposix.h>
6	.PP
7	.SM
8	.br
9	.B int regcomp(regex_t \fIpreg\fR, const char \fIpattern\fR,
10	.ti +5n
11	.B int \fIcflags\fR);
12	.PP
13	.br
14	.B int regexec(regex_t \fIpreg\fR, const char \fIstring\fR,
15	.ti +5n
16	.B size_t \fInmatch\fR, regmatch_t \fIpmatch\fR[], int \fIeflags\fR);
17	.PP
18	.br
19	.B size_t regerror(int \fIerrcode\fR, const regex_t *\fIpreg\fR,
20	.ti +5n
21	.B char *\fIerrbuf\fR, size_t \fIerrbuf_size\fR);
22	.PP
23	.br
24	.B void regfree(regex_t *\fIpreg\fR);
25
26	.SH DESCRIPTION
27	.rs
28	.sp
29	This set of functions provides a POSIX-style API to the PCRE regular expression
30	package. See the
31	.\" HREF
32	\fBpcreapi\fR
33	.\"
34	documentation for a description of the native API, which contains additional
35	functionality.
36
37	The functions described here are just wrapper functions that ultimately call
38	the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fR
39	header file, and on Unix systems the library itself is called
40	\fBpcreposix.a\fR, so can be accessed by adding \fB-lpcreposix\fR to the
41	command for linking an application which uses them. Because the POSIX functions
42	call the native ones, it is also necessary to add \fR-lpcre\fR.
43
44	I have implemented only those option bits that can be reasonably mapped to PCRE
45	native options. In addition, the options REG_EXTENDED and REG_NOSUB are defined
46	with the value zero. They have no effect, but since programs that are written
47	to the POSIX interface often use them, this makes it easier to slot in PCRE as
48	a replacement library. Other POSIX options are not even defined.
49
50	When PCRE is called via these functions, it is only the API that is POSIX-like
51	in style. The syntax and semantics of the regular expressions themselves are
52	still those of Perl, subject to the setting of various PCRE options, as
53	described below. "POSIX-like in style" means that the API approximates to the
54	POSIX definition; it is not fully POSIX-compatible, and in multi-byte encoding
55	domains it is probably even less compatible.
56
57	The header for these functions is supplied as \fBpcreposix.h\fR to avoid any
58	potential clash with other POSIX libraries. It can, of course, be renamed or
59	aliased as \fBregex.h\fR, which is the "correct" name. It provides two
60	structure types, \fIregex_t\fR for compiled internal forms, and
61	\fIregmatch_t\fR for returning captured substrings. It also defines some
62	constants whose names start with "REG_"; these are used for setting options and
63	identifying error codes.
64
65	.SH COMPILING A PATTERN
66	.rs
67	.sp
68	The function \fBregcomp()\fR is called to compile a pattern into an
69	internal form. The pattern is a C string terminated by a binary zero, and
70	is passed in the argument \fIpattern\fR. The \fIpreg\fR argument is a pointer
71	to a regex_t structure which is used as a base for storing information about
72	the compiled expression.
73
74	The argument \fIcflags\fR is either zero, or contains one or more of the bits
75	defined by the following macros:
76
77	REG_ICASE
78
79	The PCRE_CASELESS option is set when the expression is passed for compilation
80	to the native function.
81
82	REG_NEWLINE
83
84	The PCRE_MULTILINE option is set when the expression is passed for compilation
85	to the native function. Note that this does \fInot\fR mimic the defined POSIX
86	behaviour for REG_NEWLINE (see the following section).
87
88	In the absence of these flags, no options are passed to the native function.
89	This means the the regex is compiled with PCRE default semantics. In
90	particular, the way it handles newline characters in the subject string is the
91	Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only
92	\fIsome\fR of the effects specified for REG_NEWLINE. It does not affect the way
93	newlines are matched by . (they aren't) or by a negative class such as [^a]
94	(they are).
95
96	The yield of \fBregcomp()\fR is zero on success, and non-zero otherwise. The
97	\fIpreg\fR structure is filled in on success, and one member of the structure
98	is public: \fIre_nsub\fR contains the number of capturing subpatterns in
99	the regular expression. Various error codes are defined in the header file.
100
101	.SH MATCHING NEWLINE CHARACTERS
102	.rs
103	.sp
104	This area is not simple, because POSIX and Perl take different views of things.
105	It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never
106	intended to be a POSIX engine. The following table lists the different
107	possibilities for matching newline characters in PCRE:
108
109	Default Change with
110
111	. matches newline no PCRE_DOTALL
112	newline matches [^a] yes not changeable
113	$ matches \\n at end yes PCRE_DOLLARENDONLY
114	$ matches \\n in middle no PCRE_MULTILINE
115	^ matches \\n in middle no PCRE_MULTILINE
116
117	This is the equivalent table for POSIX:
118
119	Default Change with
120
121	. matches newline yes REG_NEWLINE
122	newline matches [^a] yes REG_NEWLINE
123	$ matches \\n at end no REG_NEWLINE
124	$ matches \\n in middle no REG_NEWLINE
125	^ matches \\n in middle no REG_NEWLINE
126
127	PCRE's behaviour is the same as Perl's, except that there is no equivalent for
128	PCRE_DOLLARENDONLY in Perl. In both PCRE and Perl, there is no way to stop
129	newline from matching [^a].
130
131	The default POSIX newline handling can be obtained by setting PCRE_DOTALL and
132	PCRE_DOLLARENDONLY, but there is no way to make PCRE behave exactly as for the
133	REG_NEWLINE action.
134
135	.SH MATCHING A PATTERN
136	.rs
137	.sp
138	The function \fBregexec()\fR is called to match a pre-compiled pattern
139	\fIpreg\fR against a given \fIstring\fR, which is terminated by a zero byte,
140	subject to the options in \fIeflags\fR. These can be:
141
142	REG_NOTBOL
143
144	The PCRE_NOTBOL option is set when calling the underlying PCRE matching
145	function.
146
147	REG_NOTEOL
148
149	The PCRE_NOTEOL option is set when calling the underlying PCRE matching
150	function.
151
152	The portion of the string that was matched, and also any captured substrings,
153	are returned via the \fIpmatch\fR argument, which points to an array of
154	\fInmatch\fR structures of type \fIregmatch_t\fR, containing the members
155	\fIrm_so\fR and \fIrm_eo\fR. These contain the offset to the first character of
156	each substring and the offset to the first character after the end of each
157	substring, respectively. The 0th element of the vector relates to the entire
158	portion of \fIstring\fR that was matched; subsequent elements relate to the
159	capturing subpatterns of the regular expression. Unused entries in the array
160	have both structure members set to -1.
161
162	A successful match yields a zero return; various error codes are defined in the
163	header file, of which REG_NOMATCH is the "expected" failure code.
164
165	.SH ERROR MESSAGES
166	.rs
167	.sp
168	The \fBregerror()\fR function maps a non-zero errorcode from either
169	\fBregcomp()\fR or \fBregexec()\fR to a printable message. If \fIpreg\fR is not
170	NULL, the error should have arisen from the use of that structure. A message
171	terminated by a binary zero is placed in \fIerrbuf\fR. The length of the
172	message, including the zero, is limited to \fIerrbuf_size\fR. The yield of the
173	function is the size of buffer needed to hold the whole message.
174
175	.SH STORAGE
176	.rs
177	.sp
178	Compiling a regular expression causes memory to be allocated and associated
179	with the \fIpreg\fR structure. The function \fBregfree()\fR frees all such
180	memory, after which \fIpreg\fR may no longer be used as a compiled expression.
181
182	.SH AUTHOR
183	.rs
184	.sp
185	Philip Hazel <ph10@cam.ac.uk>
186	.br
187	University Computing Service,
188	.br
189	Cambridge CB2 3QG, England.
190
191	.in 0
192	Last updated: 03 February 2003
193	.br
194	Copyright (c) 1997-2003 University of Cambridge.