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1 nigel 41 .TH PCRE 3
2     .SH NAME
3 nigel 63 PCRE - Perl-compatible regular expressions
5 nigel 63 .rs
6     .sp
7 nigel 41 The PCRE library is a set of functions that implement regular expression
8 nigel 63 pattern matching using the same syntax and semantics as Perl, with just a few
9 ph10 456 differences. Some features that appeared in Python and PCRE before they
10     appeared in Perl are also available using the Python syntax, there is some
11     support for one or two .NET and Oniguruma syntax items, and there is an option
12     for requesting some minor changes that give better JavaScript compatibility.
13 nigel 75 .P
14 ph10 456 The current implementation of PCRE corresponds approximately with Perl 5.10,
15     including support for UTF-8 encoded strings and Unicode general category
16     properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
17     is not the default. The Unicode tables correspond to Unicode release 5.1.
18 nigel 93 .P
19     In addition to the Perl-compatible matching function, PCRE contains an
20 ph10 456 alternative function that matches the same compiled patterns in a different
21     way. In certain circumstances, the alternative function has some advantages.
22     For a discussion of the two matching algorithms, see the
23 nigel 77 .\" HREF
24     \fBpcrematching\fP
25     .\"
26     page.
27     .P
28 nigel 75 PCRE is written in C and released as a C library. A number of people have
29 nigel 77 written wrappers and interfaces of various kinds. In particular, Google Inc.
30     have provided a comprehensive C++ wrapper. This is now included as part of the
31     PCRE distribution. The
32     .\" HREF
33     \fBpcrecpp\fP
34     .\"
35     page has details of this interface. Other people's contributions can be found
36     in the \fIContrib\fR directory at the primary FTP site, which is:
37 nigel 75 .sp
38 nigel 63 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
39     .\" </a>
40     ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
41 nigel 75 .P
42 nigel 63 Details of exactly which Perl regular expression features are and are not
43     supported by PCRE are given in separate documents. See the
44     .\" HREF
45     \fBpcrepattern\fR
46     .\"
47     and
48     .\" HREF
49     \fBpcrecompat\fR
50     .\"
51 ph10 208 pages. There is a syntax summary in the
52     .\" HREF
53     \fBpcresyntax\fR
54     .\"
55     page.
56 nigel 75 .P
57 nigel 63 Some features of PCRE can be included, excluded, or changed when the library is
58     built. The
59     .\" HREF
60     \fBpcre_config()\fR
61     .\"
62     function makes it possible for a client to discover which features are
63 nigel 75 available. The features themselves are described in the
64     .\" HREF
65     \fBpcrebuild\fP
66     .\"
67     page. Documentation about building PCRE for various operating systems can be
68 ph10 456 found in the \fBREADME\fP and \fBNON-UNIX-USE\fP files in the source
69     distribution.
70 nigel 77 .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 nigel 83 "_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 nigel 75 .
79     .
81 nigel 63 .rs
82     .sp
83 nigel 75 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 ph10 429 all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
87     of searching. The sections are as follows:
88 nigel 75 .sp
89 nigel 63 pcre this document
90 ph10 153 pcre-config show PCRE installation configuration information
91 nigel 77 pcreapi details of PCRE's native C API
92 nigel 63 pcrebuild options for building PCRE
93     pcrecallout details of the callout feature
94     pcrecompat discussion of Perl compatibility
95 nigel 77 pcrecpp details of the C++ wrapper
96 ph10 429 pcredemo a demonstration C program that uses PCRE
97 nigel 75 pcregrep description of the \fBpcregrep\fP command
98 nigel 77 pcrematching discussion of the two matching algorithms
99 nigel 75 pcrepartial details of the partial matching facility
100     .\" JOIN
101 nigel 63 pcrepattern syntax and semantics of supported
102     regular expressions
103     pcreperform discussion of performance issues
104 nigel 77 pcreposix the POSIX-compatible C API
105 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
106 ph10 429 pcresample discussion of the pcredemo program
107 nigel 91 pcrestack discussion of stack usage
108 ph10 456 pcresyntax quick syntax reference
109 nigel 75 pcretest description of the \fBpcretest\fP testing command
110     .sp
111 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
112 nigel 77 C library function, listing its arguments and results.
113 nigel 75 .
114     .
115 nigel 41 .SH LIMITATIONS
116 nigel 63 .rs
117     .sp
118 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
119     practice be relevant.
120 nigel 75 .P
121 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
125 nigel 63 distribution and the
126     .\" HREF
127 nigel 75 \fBpcrebuild\fP
128 nigel 63 .\"
129 nigel 75 documentation for details). In these cases the limit is substantially larger.
130 nigel 93 However, the speed of execution is slower.
131 nigel 75 .P
132 ph10 208 All values in repeating quantifiers must be less than 65536.
133 nigel 75 .P
134 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
135     no more than 65535 capturing subpatterns.
136 nigel 75 .P
137 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
138     maximum number of named subpatterns is 10000.
139 nigel 91 .P
140 nigel 41 The maximum length of a subject string is the largest positive number that an
141 nigel 77 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 nigel 91 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 ph10 211 .
151 ph10 456 .
152 nigel 63 .\" HTML <a name="utf8support"></a>
153 nigel 75 .
155 nigel 63 .rs
156     .sp
157 nigel 75 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 nigel 63 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 nigel 75 \fBpcre_compile()\fP
166 nigel 63 .\"
167 ph10 412 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 ph10 456 strings of 1-byte characters.
171 nigel 75 .P
172 nigel 49 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 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175 nigel 75 .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 nigel 87 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 nigel 75 .\" HREF
183     \fBpcrepattern\fP
184     .\"
185 nigel 87 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 ph10 211 .
190 ph10 456 .
191 ph10 211 .\" HTML <a name="utf8strings"></a>
192     .
193     .SS "Validity of UTF-8 strings"
194     .rs
195     .sp
196     When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
197     are (by default) checked for validity on entry to the relevant functions. From
198     release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
199     themselves derived from the Unicode specification. Earlier releases of PCRE
200     followed the rules of RFC 2279, which allows the full range of 31-bit values (0
201     to 0x7FFFFFFF). The current check allows only values in the range U+0 to
202     U+10FFFF, excluding U+D800 to U+DFFF.
203 nigel 75 .P
204 ph10 211 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
205     Unicode Standard says this: "The Low Surrogate Area does not contain any
206     character assignments, consequently no character code charts or namelists are
207     provided for this area. Surrogates are reserved for use with UTF-16 and then
208     must be used in pairs." The code points that are encoded by UTF-16 pairs are
209     available as independent code points in the UTF-8 encoding. (In other words,
210     the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
211     UTF-8.)
212 nigel 75 .P
213 ph10 211 If an invalid UTF-8 string is passed to PCRE, an error return
214     (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
215     your strings are valid, and therefore want to skip these checks in order to
216     improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
217     at run time, PCRE assumes that the pattern or subject it is given
218     (respectively) contains only valid UTF-8 codes. In this case, it does not
219     diagnose an invalid UTF-8 string.
220 nigel 75 .P
221 ph10 211 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
222     happens depends on why the string is invalid. If the string conforms to the
223     "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
224     in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
225     test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
226     rules of RFC 2279. However, if the string does not even conform to RFC 2279,
227     the result is undefined. Your program may crash.
228 ph10 209 .P
229 ph10 211 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
230     encoded in a UTF-8-like manner as per the old RFC, you can set
231     PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
232     situation, you will have to apply your own validity check.
233     .
234     .SS "General comments about UTF-8 mode"
235     .rs
236     .sp
237     1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
238 nigel 87 UTF-8 character if the value is greater than 127.
239 nigel 75 .P
240 ph10 211 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
241 nigel 91 characters for values greater than \e177.
242     .P
243 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
244 nigel 75 bytes, for example: \ex{100}{3}.
245     .P
246 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
247 nigel 75 .P
248 ph10 211 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
249 nigel 77 but its use can lead to some strange effects. This facility is not available in
250     the alternative matching function, \fBpcre_dfa_exec()\fP.
251 nigel 75 .P
252 ph10 211 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
253 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
254     digits, spaces, or word characters remain the same set as before, all with
255 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
256     property support, because to do otherwise would slow down PCRE in many common
257     cases. If you really want to test for a wider sense of, say, "digit", you
258 ph10 406 must use Unicode property tests such as \ep{Nd}. Note that this also applies to
259 ph10 394 \eb, because it is defined in terms of \ew and \eW.
260 nigel 75 .P
261 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
262 nigel 75 low-valued characters.
263     .P
264 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
265 ph10 181 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
266     .P
267 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
268 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
269     property support is available, PCRE still uses its own character tables when
270     checking the case of low-valued characters, so as not to degrade performance.
271     The Unicode property information is used only for characters with higher
272 nigel 87 values. Even when Unicode property support is available, PCRE supports
273     case-insensitive matching only when there is a one-to-one mapping between a
274     letter's cases. There are a small number of many-to-one mappings in Unicode;
275     these are not supported by PCRE.
276 nigel 75 .
277 ph10 99 .
278 nigel 41 .SH AUTHOR
279 nigel 63 .rs
280     .sp
281 ph10 99 .nf
282 nigel 77 Philip Hazel
283 ph10 99 University Computing Service
284 nigel 93 Cambridge CB2 3QH, England.
285 ph10 99 .fi
286 nigel 77 .P
287     Putting an actual email address here seems to have been a spam magnet, so I've
288 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
289     two digits 10, at the domain cam.ac.uk.
290 ph10 99 .
291     .
292     .SH REVISION
293     .rs
294 nigel 75 .sp
295 ph10 99 .nf
296 ph10 456 Last updated: 28 September 2009
297 ph10 394 Copyright (c) 1997-2009 University of Cambridge.
298 ph10 99 .fi


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