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Add pcredemo man page, containing a listing of pcredemo.c.
1 .TH PCRE 3
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The PCRE library is a set of functions that implement regular expression
8 pattern matching using the same syntax and semantics as Perl, with just a few
9 differences. Certain features that appeared in Python and PCRE before they
10 appeared in Perl are also available using the Python syntax. There is also some
11 support for certain .NET and Oniguruma syntax items, and there is an option for
12 requesting some minor changes that give better JavaScript compatibility.
13 .P
14 The current implementation of PCRE (release 8.xx) corresponds approximately
15 with Perl 5.10, including support for UTF-8 encoded strings and Unicode general
16 category properties. However, UTF-8 and Unicode support has to be explicitly
17 enabled; it is not the default. The Unicode tables correspond to Unicode
18 release 5.1.
19 .P
20 In addition to the Perl-compatible matching function, PCRE contains an
21 alternative matching function that matches the same compiled patterns in a
22 different way. In certain circumstances, the alternative function has some
23 advantages. For a discussion of the two matching algorithms, see the
24 .\" HREF
25 \fBpcrematching\fP
26 .\"
27 page.
28 .P
29 PCRE is written in C and released as a C library. A number of people have
30 written wrappers and interfaces of various kinds. In particular, Google Inc.
31 have provided a comprehensive C++ wrapper. This is now included as part of the
32 PCRE distribution. The
33 .\" HREF
34 \fBpcrecpp\fP
35 .\"
36 page has details of this interface. Other people's contributions can be found
37 in the \fIContrib\fR directory at the primary FTP site, which is:
38 .sp
39 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
40 .\" </a>
41 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
42 .P
43 Details of exactly which Perl regular expression features are and are not
44 supported by PCRE are given in separate documents. See the
45 .\" HREF
46 \fBpcrepattern\fR
47 .\"
48 and
49 .\" HREF
50 \fBpcrecompat\fR
51 .\"
52 pages. There is a syntax summary in the
53 .\" HREF
54 \fBpcresyntax\fR
55 .\"
56 page.
57 .P
58 Some features of PCRE can be included, excluded, or changed when the library is
59 built. The
60 .\" HREF
61 \fBpcre_config()\fR
62 .\"
63 function makes it possible for a client to discover which features are
64 available. The features themselves are described in the
65 .\" HREF
66 \fBpcrebuild\fP
67 .\"
68 page. Documentation about building PCRE for various operating systems can be
69 found in the \fBREADME\fP file in the source distribution.
70 .P
71 The library contains a number of undocumented internal functions and data
72 tables that are used by more than one of the exported external functions, but
73 which are not intended for use by external callers. Their names all begin with
74 "_pcre_", which hopefully will not provoke any name clashes. In some
75 environments, it is possible to control which external symbols are exported
76 when a shared library is built, and in these cases the undocumented symbols are
77 not exported.
78 .
79 .
81 .rs
82 .sp
83 The user documentation for PCRE comprises a number of different sections. In
84 the "man" format, each of these is a separate "man page". In the HTML format,
85 each is a separate page, linked from the index page. In the plain text format,
86 all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
87 of searching. The sections are as follows:
88 .sp
89 pcre this document
90 pcre-config show PCRE installation configuration information
91 pcreapi details of PCRE's native C API
92 pcrebuild options for building PCRE
93 pcrecallout details of the callout feature
94 pcrecompat discussion of Perl compatibility
95 pcrecpp details of the C++ wrapper
96 pcredemo a demonstration C program that uses PCRE
97 pcregrep description of the \fBpcregrep\fP command
98 pcrematching discussion of the two matching algorithms
99 pcrepartial details of the partial matching facility
100 .\" JOIN
101 pcrepattern syntax and semantics of supported
102 regular expressions
103 pcresyntax quick syntax reference
104 pcreperform discussion of performance issues
105 pcreposix the POSIX-compatible C API
106 pcreprecompile details of saving and re-using precompiled patterns
107 pcresample discussion of the pcredemo program
108 pcrestack discussion of stack usage
109 pcretest description of the \fBpcretest\fP testing command
110 .sp
111 In addition, in the "man" and HTML formats, there is a short page for each
112 C library function, listing its arguments and results.
113 .
114 .
116 .rs
117 .sp
118 There are some size limitations in PCRE but it is hoped that they will never in
119 practice be relevant.
120 .P
121 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
122 compiled with the default internal linkage size of 2. If you want to process
123 regular expressions that are truly enormous, you can compile PCRE with an
124 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
125 distribution and the
126 .\" HREF
127 \fBpcrebuild\fP
128 .\"
129 documentation for details). In these cases the limit is substantially larger.
130 However, the speed of execution is slower.
131 .P
132 All values in repeating quantifiers must be less than 65536.
133 .P
134 There is no limit to the number of parenthesized subpatterns, but there can be
135 no more than 65535 capturing subpatterns.
136 .P
137 The maximum length of name for a named subpattern is 32 characters, and the
138 maximum number of named subpatterns is 10000.
139 .P
140 The maximum length of a subject string is the largest positive number that an
141 integer variable can hold. However, when using the traditional matching
142 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
143 This means that the available stack space may limit the size of a subject
144 string that can be processed by certain patterns. For a discussion of stack
145 issues, see the
146 .\" HREF
147 \fBpcrestack\fP
148 .\"
149 documentation.
150 .
151 .\" HTML <a name="utf8support"></a>
152 .
153 .
155 .rs
156 .sp
157 From release 3.3, PCRE has had some support for character strings encoded in
158 the UTF-8 format. For release 4.0 this was greatly extended to cover most
159 common requirements, and in release 5.0 additional support for Unicode general
160 category properties was added.
161 .P
162 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
163 the code, and, in addition, you must call
164 .\" HREF
165 \fBpcre_compile()\fP
166 .\"
167 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
168 (*UTF8). When either of these is the case, both the pattern and any subject
169 strings that are matched against it are treated as UTF-8 strings instead of
170 just strings of bytes.
171 .P
172 If you compile PCRE with UTF-8 support, but do not use it at run time, the
173 library will be a bit bigger, but the additional run time overhead is limited
174 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175 .P
176 If PCRE is built with Unicode character property support (which implies UTF-8
177 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
178 The available properties that can be tested are limited to the general
179 category properties such as Lu for an upper case letter or Nd for a decimal
180 number, the Unicode script names such as Arabic or Han, and the derived
181 properties Any and L&. A full list is given in the
182 .\" HREF
183 \fBpcrepattern\fP
184 .\"
185 documentation. Only the short names for properties are supported. For example,
186 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
187 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
188 compatibility with Perl 5.6. PCRE does not support this.
189 .
190 .\" HTML <a name="utf8strings"></a>
191 .
192 .SS "Validity of UTF-8 strings"
193 .rs
194 .sp
195 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
196 are (by default) checked for validity on entry to the relevant functions. From
197 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
198 themselves derived from the Unicode specification. Earlier releases of PCRE
199 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
200 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
201 U+10FFFF, excluding U+D800 to U+DFFF.
202 .P
203 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
204 Unicode Standard says this: "The Low Surrogate Area does not contain any
205 character assignments, consequently no character code charts or namelists are
206 provided for this area. Surrogates are reserved for use with UTF-16 and then
207 must be used in pairs." The code points that are encoded by UTF-16 pairs are
208 available as independent code points in the UTF-8 encoding. (In other words,
209 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
210 UTF-8.)
211 .P
212 If an invalid UTF-8 string is passed to PCRE, an error return
213 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
214 your strings are valid, and therefore want to skip these checks in order to
215 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
216 at run time, PCRE assumes that the pattern or subject it is given
217 (respectively) contains only valid UTF-8 codes. In this case, it does not
218 diagnose an invalid UTF-8 string.
219 .P
220 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
221 happens depends on why the string is invalid. If the string conforms to the
222 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
223 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
224 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
225 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
226 the result is undefined. Your program may crash.
227 .P
228 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
229 encoded in a UTF-8-like manner as per the old RFC, you can set
230 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
231 situation, you will have to apply your own validity check.
232 .
233 .SS "General comments about UTF-8 mode"
234 .rs
235 .sp
236 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
237 UTF-8 character if the value is greater than 127.
238 .P
239 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
240 characters for values greater than \e177.
241 .P
242 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
243 bytes, for example: \ex{100}{3}.
244 .P
245 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
246 .P
247 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
248 but its use can lead to some strange effects. This facility is not available in
249 the alternative matching function, \fBpcre_dfa_exec()\fP.
250 .P
251 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
252 test characters of any code value, but the characters that PCRE recognizes as
253 digits, spaces, or word characters remain the same set as before, all with
254 values less than 256. This remains true even when PCRE includes Unicode
255 property support, because to do otherwise would slow down PCRE in many common
256 cases. If you really want to test for a wider sense of, say, "digit", you
257 must use Unicode property tests such as \ep{Nd}. Note that this also applies to
258 \eb, because it is defined in terms of \ew and \eW.
259 .P
260 7. Similarly, characters that match the POSIX named character classes are all
261 low-valued characters.
262 .P
263 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
264 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
265 .P
266 9. Case-insensitive matching applies only to characters whose values are less
267 than 128, unless PCRE is built with Unicode property support. Even when Unicode
268 property support is available, PCRE still uses its own character tables when
269 checking the case of low-valued characters, so as not to degrade performance.
270 The Unicode property information is used only for characters with higher
271 values. Even when Unicode property support is available, PCRE supports
272 case-insensitive matching only when there is a one-to-one mapping between a
273 letter's cases. There are a small number of many-to-one mappings in Unicode;
274 these are not supported by PCRE.
275 .
276 .
278 .rs
279 .sp
280 .nf
281 Philip Hazel
282 University Computing Service
283 Cambridge CB2 3QH, England.
284 .fi
285 .P
286 Putting an actual email address here seems to have been a spam magnet, so I've
287 taken it away. If you want to email me, use my two initials, followed by the
288 two digits 10, at the domain cam.ac.uk.
289 .
290 .
292 .rs
293 .sp
294 .nf
295 Last updated: 01 September 2009
296 Copyright (c) 1997-2009 University of Cambridge.
297 .fi


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