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1 .TH PCRE 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions.
4 .SH "SYNOPSIS OF POSIX API"
5 .rs
6 .sp
7 .B #include <pcreposix.h>
8 .PP
9 .SM
10 .br
11 .B int regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
12 .ti +5n
13 .B int \fIcflags\fP);
14 .PP
15 .br
16 .B int regexec(regex_t *\fIpreg\fP, const char *\fIstring\fP,
17 .ti +5n
18 .B size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);
19 .PP
20 .br
21 .B size_t regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,
22 .ti +5n
23 .B char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);
24 .PP
25 .br
26 .B void regfree(regex_t *\fIpreg\fP);
27 .
28 .SH DESCRIPTION
29 .rs
30 .sp
31 This set of functions provides a POSIX-style API to the PCRE regular expression
32 package. See the
33 .\" HREF
34 \fBpcreapi\fP
35 .\"
36 documentation for a description of PCRE's native API, which contains additional
37 functionality.
38 .P
39 The functions described here are just wrapper functions that ultimately call
40 the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fP
41 header file, and on Unix systems the library itself is called
42 \fBpcreposix.a\fP, so can be accessed by adding \fB-lpcreposix\fP to the
43 command for linking an application that uses them. Because the POSIX functions
44 call the native ones, it is also necessary to add \fB-lpcre\fP.
45 .P
46 I have implemented only those option bits that can be reasonably mapped to PCRE
47 native options. In addition, the options REG_EXTENDED and REG_NOSUB are defined
48 with the value zero. They have no effect, but since programs that are written
49 to the POSIX interface often use them, this makes it easier to slot in PCRE as
50 a replacement library. Other POSIX options are not even defined.
51 .P
52 When PCRE is called via these functions, it is only the API that is POSIX-like
53 in style. The syntax and semantics of the regular expressions themselves are
54 still those of Perl, subject to the setting of various PCRE options, as
55 described below. "POSIX-like in style" means that the API approximates to the
56 POSIX definition; it is not fully POSIX-compatible, and in multi-byte encoding
57 domains it is probably even less compatible.
58 .P
59 The header for these functions is supplied as \fBpcreposix.h\fP to avoid any
60 potential clash with other POSIX libraries. It can, of course, be renamed or
61 aliased as \fBregex.h\fP, which is the "correct" name. It provides two
62 structure types, \fIregex_t\fP for compiled internal forms, and
63 \fIregmatch_t\fP for returning captured substrings. It also defines some
64 constants whose names start with "REG_"; these are used for setting options and
65 identifying error codes.
66 .P
67 .SH "COMPILING A PATTERN"
68 .rs
69 .sp
70 The function \fBregcomp()\fP is called to compile a pattern into an
71 internal form. The pattern is a C string terminated by a binary zero, and
72 is passed in the argument \fIpattern\fP. The \fIpreg\fP argument is a pointer
73 to a \fBregex_t\fP structure that is used as a base for storing information
74 about the compiled expression.
75 .P
76 The argument \fIcflags\fP is either zero, or contains one or more of the bits
77 defined by the following macros:
78 .sp
79 REG_ICASE
80 .sp
81 The PCRE_CASELESS option is set when the expression is passed for compilation
82 to the native function.
83 .sp
84 REG_NEWLINE
85 .sp
86 The PCRE_MULTILINE option is set when the expression is passed for compilation
87 to the native function. Note that this does \fInot\fP mimic the defined POSIX
88 behaviour for REG_NEWLINE (see the following section).
89 .P
90 In the absence of these flags, no options are passed to the native function.
91 This means the the regex is compiled with PCRE default semantics. In
92 particular, the way it handles newline characters in the subject string is the
93 Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only
94 \fIsome\fP of the effects specified for REG_NEWLINE. It does not affect the way
95 newlines are matched by . (they aren't) or by a negative class such as [^a]
96 (they are).
97 .P
98 The yield of \fBregcomp()\fP is zero on success, and non-zero otherwise. The
99 \fIpreg\fP structure is filled in on success, and one member of the structure
100 is public: \fIre_nsub\fP contains the number of capturing subpatterns in
101 the regular expression. Various error codes are defined in the header file.
102 .
103 .
104 .SH "MATCHING NEWLINE CHARACTERS"
105 .rs
106 .sp
107 This area is not simple, because POSIX and Perl take different views of things.
108 It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never
109 intended to be a POSIX engine. The following table lists the different
110 possibilities for matching newline characters in PCRE:
111 .sp
112 Default Change with
113 .sp
114 . matches newline no PCRE_DOTALL
115 newline matches [^a] yes not changeable
116 $ matches \en at end yes PCRE_DOLLARENDONLY
117 $ matches \en in middle no PCRE_MULTILINE
118 ^ matches \en in middle no PCRE_MULTILINE
119 .sp
120 This is the equivalent table for POSIX:
121 .sp
122 Default Change with
123 .sp
124 . matches newline yes REG_NEWLINE
125 newline matches [^a] yes REG_NEWLINE
126 $ matches \en at end no REG_NEWLINE
127 $ matches \en in middle no REG_NEWLINE
128 ^ matches \en in middle no REG_NEWLINE
129 .sp
130 PCRE's behaviour is the same as Perl's, except that there is no equivalent for
131 PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop
132 newline from matching [^a].
133 .P
134 The default POSIX newline handling can be obtained by setting PCRE_DOTALL and
135 PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the
136 REG_NEWLINE action.
137 .
138 .
139 .SH "MATCHING A PATTERN"
140 .rs
141 .sp
142 The function \fBregexec()\fP is called to match a compiled pattern \fIpreg\fP
143 against a given \fIstring\fP, which is terminated by a zero byte, subject to
144 the options in \fIeflags\fP. These can be:
145 .sp
146 REG_NOTBOL
147 .sp
148 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
149 function.
150 .sp
151 REG_NOTEOL
152 .sp
153 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
154 function.
155 .P
156 The portion of the string that was matched, and also any captured substrings,
157 are returned via the \fIpmatch\fP argument, which points to an array of
158 \fInmatch\fP structures of type \fIregmatch_t\fP, containing the members
159 \fIrm_so\fP and \fIrm_eo\fP. These contain the offset to the first character of
160 each substring and the offset to the first character after the end of each
161 substring, respectively. The 0th element of the vector relates to the entire
162 portion of \fIstring\fP that was matched; subsequent elements relate to the
163 capturing subpatterns of the regular expression. Unused entries in the array
164 have both structure members set to -1.
165 .P
166 A successful match yields a zero return; various error codes are defined in the
167 header file, of which REG_NOMATCH is the "expected" failure code.
168 .
169 .
170 .SH "ERROR MESSAGES"
171 .rs
172 .sp
173 The \fBregerror()\fP function maps a non-zero errorcode from either
174 \fBregcomp()\fP or \fBregexec()\fP to a printable message. If \fIpreg\fP is not
175 NULL, the error should have arisen from the use of that structure. A message
176 terminated by a binary zero is placed in \fIerrbuf\fP. The length of the
177 message, including the zero, is limited to \fIerrbuf_size\fP. The yield of the
178 function is the size of buffer needed to hold the whole message.
179 .
180 .
181 .SH MEMORY USAGE
182 .rs
183 .sp
184 Compiling a regular expression causes memory to be allocated and associated
185 with the \fIpreg\fP structure. The function \fBregfree()\fP frees all such
186 memory, after which \fIpreg\fP may no longer be used as a compiled expression.
187 .
188 .
189 .SH AUTHOR
190 .rs
191 .sp
192 Philip Hazel <ph10@cam.ac.uk>
193 .br
194 University Computing Service,
195 .br
196 Cambridge CB2 3QG, England.
197 .P
198 .in 0
199 Last updated: 07 September 2004
200 .br
201 Copyright (c) 1997-2004 University of Cambridge.

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