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Revision 424 - (show annotations)
Sat Aug 15 18:45:00 2009 UTC (11 years, 3 months ago) by ph10
File size: 9781 byte(s)
Add warning comment to pcreposix doc.
3 PCRE - Perl-compatible regular expressions.
5 .rs
6 .sp
7 .B #include <pcreposix.h>
8 .PP
9 .SM
10 .B int regcomp(regex_t *\fIpreg\fP, const char *\fIpattern\fP,
11 .ti +5n
12 .B int \fIcflags\fP);
13 .PP
14 .B int regexec(regex_t *\fIpreg\fP, const char *\fIstring\fP,
15 .ti +5n
16 .B size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[], int \fIeflags\fP);
17 .PP
18 .B size_t regerror(int \fIerrcode\fP, const regex_t *\fIpreg\fP,
19 .ti +5n
20 .B char *\fIerrbuf\fP, size_t \fIerrbuf_size\fP);
21 .PP
22 .B void regfree(regex_t *\fIpreg\fP);
23 .
25 .rs
26 .sp
27 This set of functions provides a POSIX-style API to the PCRE regular expression
28 package. See the
29 .\" HREF
30 \fBpcreapi\fP
31 .\"
32 documentation for a description of PCRE's native API, which contains much
33 additional functionality.
34 .P
35 The functions described here are just wrapper functions that ultimately call
36 the PCRE native API. Their prototypes are defined in the \fBpcreposix.h\fP
37 header file, and on Unix systems the library itself is called
38 \fBpcreposix.a\fP, so can be accessed by adding \fB-lpcreposix\fP to the
39 command for linking an application that uses them. Because the POSIX functions
40 call the native ones, it is also necessary to add \fB-lpcre\fP.
41 .P
42 I have implemented only those POSIX option bits that can be reasonably mapped
43 to PCRE native options. In addition, the option REG_EXTENDED is defined with
44 the value zero. This has no effect, but since programs that are written to the
45 POSIX interface often use it, this makes it easier to slot in PCRE as a
46 replacement library. Other POSIX options are not even defined.
47 .P
48 When PCRE is called via these functions, it is only the API that is POSIX-like
49 in style. The syntax and semantics of the regular expressions themselves are
50 still those of Perl, subject to the setting of various PCRE options, as
51 described below. "POSIX-like in style" means that the API approximates to the
52 POSIX definition; it is not fully POSIX-compatible, and in multi-byte encoding
53 domains it is probably even less compatible.
54 .P
55 The header for these functions is supplied as \fBpcreposix.h\fP to avoid any
56 potential clash with other POSIX libraries. It can, of course, be renamed or
57 aliased as \fBregex.h\fP, which is the "correct" name. It provides two
58 structure types, \fIregex_t\fP for compiled internal forms, and
59 \fIregmatch_t\fP 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 .P
64 .rs
65 .sp
66 The function \fBregcomp()\fP 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\fP. The \fIpreg\fP argument is a pointer
69 to a \fBregex_t\fP structure that is used as a base for storing information
70 about the compiled regular expression.
71 .P
72 The argument \fIcflags\fP is either zero, or contains one or more of the bits
73 defined by the following macros:
74 .sp
76 .sp
77 The PCRE_DOTALL option is set when the regular expression is passed for
78 compilation to the native function. Note that REG_DOTALL is not part of the
79 POSIX standard.
80 .sp
82 .sp
83 The PCRE_CASELESS option is set when the regular expression is passed for
84 compilation to the native function.
85 .sp
87 .sp
88 The PCRE_MULTILINE option is set when the regular expression is passed for
89 compilation to the native function. Note that this does \fInot\fP mimic the
90 defined POSIX behaviour for REG_NEWLINE (see the following section).
91 .sp
93 .sp
94 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed
95 for compilation to the native function. In addition, when a pattern that is
96 compiled with this flag is passed to \fBregexec()\fP for matching, the
97 \fInmatch\fP and \fIpmatch\fP arguments are ignored, and no captured strings
98 are returned.
99 .sp
100 REG_UTF8
101 .sp
102 The PCRE_UTF8 option is set when the regular expression is passed for
103 compilation to the native function. This causes the pattern itself and all data
104 strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF8
105 is not part of the POSIX standard.
106 .P
107 In the absence of these flags, no options are passed to the native function.
108 This means the the regex is compiled with PCRE default semantics. In
109 particular, the way it handles newline characters in the subject string is the
110 Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only
111 \fIsome\fP of the effects specified for REG_NEWLINE. It does not affect the way
112 newlines are matched by . (they aren't) or by a negative class such as [^a]
113 (they are).
114 .P
115 The yield of \fBregcomp()\fP is zero on success, and non-zero otherwise. The
116 \fIpreg\fP structure is filled in on success, and one member of the structure
117 is public: \fIre_nsub\fP contains the number of capturing subpatterns in
118 the regular expression. Various error codes are defined in the header file.
119 .P
120 NOTE: If the yield of \fBregcomp()\fP is non-zero, you must not attempt to
121 use the contents of the \fIpreg\fP structure. If, for example, you pass it to
122 \fBregexec()\fP, the result is undefined and your program is likely to crash.
123 .
124 .
126 .rs
127 .sp
128 This area is not simple, because POSIX and Perl take different views of things.
129 It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never
130 intended to be a POSIX engine. The following table lists the different
131 possibilities for matching newline characters in PCRE:
132 .sp
133 Default Change with
134 .sp
135 . matches newline no PCRE_DOTALL
136 newline matches [^a] yes not changeable
137 $ matches \en at end yes PCRE_DOLLARENDONLY
138 $ matches \en in middle no PCRE_MULTILINE
139 ^ matches \en in middle no PCRE_MULTILINE
140 .sp
141 This is the equivalent table for POSIX:
142 .sp
143 Default Change with
144 .sp
145 . matches newline yes REG_NEWLINE
146 newline matches [^a] yes REG_NEWLINE
147 $ matches \en at end no REG_NEWLINE
148 $ matches \en in middle no REG_NEWLINE
149 ^ matches \en in middle no REG_NEWLINE
150 .sp
151 PCRE's behaviour is the same as Perl's, except that there is no equivalent for
152 PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop
153 newline from matching [^a].
154 .P
155 The default POSIX newline handling can be obtained by setting PCRE_DOTALL and
156 PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the
157 REG_NEWLINE action.
158 .
159 .
161 .rs
162 .sp
163 The function \fBregexec()\fP is called to match a compiled pattern \fIpreg\fP
164 against a given \fIstring\fP, which is by default terminated by a zero byte
165 (but see REG_STARTEND below), subject to the options in \fIeflags\fP. These can
166 be:
167 .sp
169 .sp
170 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
171 function.
172 .sp
174 .sp
175 The PCRE_NOTEMPTY option is set when calling the underlying PCRE matching
176 function. Note that REG_NOTEMPTY is not part of the POSIX standard. However,
177 setting this option can give more POSIX-like behaviour in some situations.
178 .sp
180 .sp
181 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
182 function.
183 .sp
185 .sp
186 The string is considered to start at \fIstring\fP + \fIpmatch[0].rm_so\fP and
187 to have a terminating NUL located at \fIstring\fP + \fIpmatch[0].rm_eo\fP
188 (there need not actually be a NUL at that location), regardless of the value of
189 \fInmatch\fP. This is a BSD extension, compatible with but not specified by
190 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software
191 intended to be portable to other systems. Note that a non-zero \fIrm_so\fP does
192 not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not
193 how it is matched.
194 .P
195 If the pattern was compiled with the REG_NOSUB flag, no data about any matched
196 strings is returned. The \fInmatch\fP and \fIpmatch\fP arguments of
197 \fBregexec()\fP are ignored.
198 .P
199 Otherwise,the portion of the string that was matched, and also any captured
200 substrings, are returned via the \fIpmatch\fP argument, which points to an
201 array of \fInmatch\fP structures of type \fIregmatch_t\fP, containing the
202 members \fIrm_so\fP and \fIrm_eo\fP. These contain the offset to the first
203 character of each substring and the offset to the first character after the end
204 of each substring, respectively. The 0th element of the vector relates to the
205 entire portion of \fIstring\fP that was matched; subsequent elements relate to
206 the capturing subpatterns of the regular expression. Unused entries in the
207 array have both structure members set to -1.
208 .P
209 A successful match yields a zero return; various error codes are defined in the
210 header file, of which REG_NOMATCH is the "expected" failure code.
211 .
212 .
214 .rs
215 .sp
216 The \fBregerror()\fP function maps a non-zero errorcode from either
217 \fBregcomp()\fP or \fBregexec()\fP to a printable message. If \fIpreg\fP is not
218 NULL, the error should have arisen from the use of that structure. A message
219 terminated by a binary zero is placed in \fIerrbuf\fP. The length of the
220 message, including the zero, is limited to \fIerrbuf_size\fP. The yield of the
221 function is the size of buffer needed to hold the whole message.
222 .
223 .
225 .rs
226 .sp
227 Compiling a regular expression causes memory to be allocated and associated
228 with the \fIpreg\fP structure. The function \fBregfree()\fP frees all such
229 memory, after which \fIpreg\fP may no longer be used as a compiled expression.
230 .
231 .
233 .rs
234 .sp
235 .nf
236 Philip Hazel
237 University Computing Service
238 Cambridge CB2 3QH, England.
239 .fi
240 .
241 .
243 .rs
244 .sp
245 .nf
246 Last updated: 15 August 2009
247 Copyright (c) 1997-2009 University of Cambridge.
248 .fi


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