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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.
54
55 The header for these functions is supplied as \fBpcreposix.h\fR to avoid any
56 potential clash with other POSIX libraries. It can, of course, be renamed or
57 aliased as \fBregex.h\fR, which is the "correct" name. It provides two
58 structure types, \fIregex_t\fR for compiled internal forms, and
59 \fIregmatch_t\fR for returning captured substrings. It also defines some
60 constants whose names start with "REG_"; these are used for setting options and
61 identifying error codes.
62
63 .SH COMPILING A PATTERN
64 .rs
65 .sp
66 The function \fBregcomp()\fR is called to compile a pattern into an
67 internal form. The pattern is a C string terminated by a binary zero, and
68 is passed in the argument \fIpattern\fR. The \fIpreg\fR argument is a pointer
69 to a regex_t structure which is used as a base for storing information about
70 the compiled expression.
71
72 The argument \fIcflags\fR is either zero, or contains one or more of the bits
73 defined by the following macros:
74
75 REG_ICASE
76
77 The PCRE_CASELESS option is set when the expression is passed for compilation
78 to the native function.
79
80 REG_NEWLINE
81
82 The PCRE_MULTILINE option is set when the expression is passed for compilation
83 to the native function. Note that this does \fInot\fR mimic the defined POSIX
84 behaviour for REG_NEWLINE (see the following section).
85
86 In the absence of these flags, no options are passed to the native function.
87 This means the the regex is compiled with PCRE default semantics. In
88 particular, the way it handles newline characters in the subject string is the
89 Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only
90 \fIsome\fR of the effects specified for REG_NEWLINE. It does not affect the way
91 newlines are matched by . (they aren't) or by a negative class such as [^a]
92 (they are).
93
94 The yield of \fBregcomp()\fR is zero on success, and non-zero otherwise. The
95 \fIpreg\fR structure is filled in on success, and one member of the structure
96 is public: \fIre_nsub\fR contains the number of capturing subpatterns in
97 the regular expression. Various error codes are defined in the header file.
98
99 .SH MATCHING NEWLINE CHARACTERS
100 .rs
101 .sp
102 This area is not simple, because POSIX and Perl take different views of things.
103 It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never
104 intended to be a POSIX engine. The following table lists the different
105 possibilities for matching newline characters in PCRE:
106
107 Default Change with
108
109 . matches newline no PCRE_DOTALL
110 newline matches [^a] yes not changeable
111 $ matches \\n at end yes PCRE_DOLLARENDONLY
112 $ matches \\n in middle no PCRE_MULTILINE
113 ^ matches \\n in middle no PCRE_MULTILINE
114
115 This is the equivalent table for POSIX:
116
117 Default Change with
118
119 . matches newline yes REG_NEWLINE
120 newline matches [^a] yes REG_NEWLINE
121 $ matches \\n at end no REG_NEWLINE
122 $ matches \\n in middle no REG_NEWLINE
123 ^ matches \\n in middle no REG_NEWLINE
124
125 PCRE's behaviour is the same as Perl's, except that there is no equivalent for
126 PCRE_DOLLARENDONLY in Perl. In both PCRE and Perl, there is no way to stop
127 newline from matching [^a].
128
129 The default POSIX newline handling can be obtained by setting PCRE_DOTALL and
130 PCRE_DOLLARENDONLY, but there is no way to make PCRE behave exactly as for the
131 REG_NEWLINE action.
132
133 .SH MATCHING A PATTERN
134 .rs
135 .sp
136 The function \fBregexec()\fR is called to match a pre-compiled pattern
137 \fIpreg\fR against a given \fIstring\fR, which is terminated by a zero byte,
138 subject to the options in \fIeflags\fR. These can be:
139
140 REG_NOTBOL
141
142 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
143 function.
144
145 REG_NOTEOL
146
147 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
148 function.
149
150 The portion of the string that was matched, and also any captured substrings,
151 are returned via the \fIpmatch\fR argument, which points to an array of
152 \fInmatch\fR structures of type \fIregmatch_t\fR, containing the members
153 \fIrm_so\fR and \fIrm_eo\fR. These contain the offset to the first character of
154 each substring and the offset to the first character after the end of each
155 substring, respectively. The 0th element of the vector relates to the entire
156 portion of \fIstring\fR that was matched; subsequent elements relate to the
157 capturing subpatterns of the regular expression. Unused entries in the array
158 have both structure members set to -1.
159
160 A successful match yields a zero return; various error codes are defined in the
161 header file, of which REG_NOMATCH is the "expected" failure code.
162
163 .SH ERROR MESSAGES
164 .rs
165 .sp
166 The \fBregerror()\fR function maps a non-zero errorcode from either
167 \fBregcomp()\fR or \fBregexec()\fR to a printable message. If \fIpreg\fR is not
168 NULL, the error should have arisen from the use of that structure. A message
169 terminated by a binary zero is placed in \fIerrbuf\fR. The length of the
170 message, including the zero, is limited to \fIerrbuf_size\fR. The yield of the
171 function is the size of buffer needed to hold the whole message.
172
173 .SH STORAGE
174 .rs
175 .sp
176 Compiling a regular expression causes memory to be allocated and associated
177 with the \fIpreg\fR structure. The function \fBregfree()\fR frees all such
178 memory, after which \fIpreg\fR may no longer be used as a compiled expression.
179
180 .SH AUTHOR
181 .rs
182 .sp
183 Philip Hazel <ph10@cam.ac.uk>
184 .br
185 University Computing Service,
186 .br
187 Cambridge CB2 3QG, England.
188
189 .in 0
190 Last updated: 03 February 2003
191 .br
192 Copyright (c) 1997-2003 University of Cambridge.

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