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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.

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