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Update UTF-8 validity check and documentation.
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.)
11 .P
12 The current implementation of PCRE (release 7.x) corresponds approximately with
13 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
14 category properties. However, UTF-8 and Unicode support has to be explicitly
15 enabled; it is not the default. The Unicode tables correspond to Unicode
16 release 5.0.0.
17 .P
18 In addition to the Perl-compatible matching function, PCRE contains an
19 alternative matching function that matches the same compiled patterns in a
20 different way. In certain circumstances, the alternative function has some
21 advantages. For a discussion of the two matching algorithms, see the
22 .\" HREF
23 \fBpcrematching\fP
24 .\"
25 page.
26 .P
27 PCRE is written in C and released as a C library. A number of people have
28 written wrappers and interfaces of various kinds. In particular, Google Inc.
29 have provided a comprehensive C++ wrapper. This is now included as part of the
30 PCRE distribution. The
31 .\" HREF
32 \fBpcrecpp\fP
33 .\"
34 page has details of this interface. Other people's contributions can be found
35 in the \fIContrib\fR directory at the primary FTP site, which is:
36 .sp
37 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
38 .\" </a>
39 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40 .P
41 Details of exactly which Perl regular expression features are and are not
42 supported by PCRE are given in separate documents. See the
43 .\" HREF
44 \fBpcrepattern\fR
45 .\"
46 and
47 .\" HREF
48 \fBpcrecompat\fR
49 .\"
50 pages. There is a syntax summary in the
51 .\" HREF
52 \fBpcresyntax\fR
53 .\"
54 page.
55 .P
56 Some features of PCRE can be included, excluded, or changed when the library is
57 built. The
58 .\" HREF
59 \fBpcre_config()\fR
60 .\"
61 function makes it possible for a client to discover which features are
62 available. The features themselves are described in the
63 .\" HREF
64 \fBpcrebuild\fP
65 .\"
66 page. Documentation about building PCRE for various operating systems can be
67 found in the \fBREADME\fP file in the source distribution.
68 .P
69 The library contains a number of undocumented internal functions and data
70 tables that are used by more than one of the exported external functions, but
71 which are not intended for use by external callers. Their names all begin with
72 "_pcre_", which hopefully will not provoke any name clashes. In some
73 environments, it is possible to control which external symbols are exported
74 when a shared library is built, and in these cases the undocumented symbols are
75 not exported.
76 .
77 .
79 .rs
80 .sp
81 The user documentation for PCRE comprises a number of different sections. In
82 the "man" format, each of these is a separate "man page". In the HTML format,
83 each is a separate page, linked from the index page. In the plain text format,
84 all the sections are concatenated, for ease of searching. The sections are as
85 follows:
86 .sp
87 pcre this document
88 pcre-config show PCRE installation configuration information
89 pcreapi details of PCRE's native C API
90 pcrebuild options for building PCRE
91 pcrecallout details of the callout feature
92 pcrecompat discussion of Perl compatibility
93 pcrecpp details of the C++ wrapper
94 pcregrep description of the \fBpcregrep\fP command
95 pcrematching discussion of the two matching algorithms
96 pcrepartial details of the partial matching facility
97 .\" JOIN
98 pcrepattern syntax and semantics of supported
99 regular expressions
100 pcresyntax quick syntax reference
101 pcreperform discussion of performance issues
102 pcreposix the POSIX-compatible C API
103 pcreprecompile details of saving and re-using precompiled patterns
104 pcresample discussion of the sample program
105 pcrestack discussion of stack usage
106 pcretest description of the \fBpcretest\fP testing command
107 .sp
108 In addition, in the "man" and HTML formats, there is a short page for each
109 C library function, listing its arguments and results.
110 .
111 .
113 .rs
114 .sp
115 There are some size limitations in PCRE but it is hoped that they will never in
116 practice be relevant.
117 .P
118 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
119 compiled with the default internal linkage size of 2. If you want to process
120 regular expressions that are truly enormous, you can compile PCRE with an
121 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
122 distribution and the
123 .\" HREF
124 \fBpcrebuild\fP
125 .\"
126 documentation for details). In these cases the limit is substantially larger.
127 However, the speed of execution is slower.
128 .P
129 All values in repeating quantifiers must be less than 65536.
130 .P
131 There is no limit to the number of parenthesized subpatterns, but there can be
132 no more than 65535 capturing subpatterns.
133 .P
134 The maximum length of name for a named subpattern is 32 characters, and the
135 maximum number of named subpatterns is 10000.
136 .P
137 The maximum length of a subject string is the largest positive number that an
138 integer variable can hold. However, when using the traditional matching
139 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
140 This means that the available stack space may limit the size of a subject
141 string that can be processed by certain patterns. For a discussion of stack
142 issues, see the
143 .\" HREF
144 \fBpcrestack\fP
145 .\"
146 documentation.
147 .
148 .\" HTML <a name="utf8support"></a>
149 .
150 .
152 .rs
153 .sp
154 From release 3.3, PCRE has had some support for character strings encoded in
155 the UTF-8 format. For release 4.0 this was greatly extended to cover most
156 common requirements, and in release 5.0 additional support for Unicode general
157 category properties was added.
158 .P
159 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
160 the code, and, in addition, you must call
161 .\" HREF
162 \fBpcre_compile()\fP
163 .\"
164 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
165 subject strings that are matched against it are treated as UTF-8 strings
166 instead of just strings of bytes.
167 .P
168 If you compile PCRE with UTF-8 support, but do not use it at run time, the
169 library will be a bit bigger, but the additional run time overhead is limited
170 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
171 .P
172 If PCRE is built with Unicode character property support (which implies UTF-8
173 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
174 The available properties that can be tested are limited to the general
175 category properties such as Lu for an upper case letter or Nd for a decimal
176 number, the Unicode script names such as Arabic or Han, and the derived
177 properties Any and L&. A full list is given in the
178 .\" HREF
179 \fBpcrepattern\fP
180 .\"
181 documentation. Only the short names for properties are supported. For example,
182 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
183 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
184 compatibility with Perl 5.6. PCRE does not support this.
185 .
186 .\" HTML <a name="utf8strings"></a>
187 .
188 .SS "Validity of UTF-8 strings"
189 .rs
190 .sp
191 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
192 are (by default) checked for validity on entry to the relevant functions. From
193 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
194 themselves derived from the Unicode specification. Earlier releases of PCRE
195 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
196 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
197 U+10FFFF, excluding U+D800 to U+DFFF.
198 .P
199 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
200 Unicode Standard says this: "The Low Surrogate Area does not contain any
201 character assignments, consequently no character code charts or namelists are
202 provided for this area. Surrogates are reserved for use with UTF-16 and then
203 must be used in pairs." The code points that are encoded by UTF-16 pairs are
204 available as independent code points in the UTF-8 encoding. (In other words,
205 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
206 UTF-8.)
207 .P
208 If an invalid UTF-8 string is passed to PCRE, an error return
209 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
210 your strings are valid, and therefore want to skip these checks in order to
211 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
212 at run time, PCRE assumes that the pattern or subject it is given
213 (respectively) contains only valid UTF-8 codes. In this case, it does not
214 diagnose an invalid UTF-8 string.
215 .P
216 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
217 happens depends on why the string is invalid. If the string conforms to the
218 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
219 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
220 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
221 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
222 the result is undefined. Your program may crash.
223 .P
224 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
225 encoded in a UTF-8-like manner as per the old RFC, you can set
226 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
227 situation, you will have to apply your own validity check.
228 .
229 .SS "General comments about UTF-8 mode"
230 .rs
231 .sp
232 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
233 UTF-8 character if the value is greater than 127.
234 .P
235 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
236 characters for values greater than \e177.
237 .P
238 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
239 bytes, for example: \ex{100}{3}.
240 .P
241 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
242 .P
243 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
244 but its use can lead to some strange effects. This facility is not available in
245 the alternative matching function, \fBpcre_dfa_exec()\fP.
246 .P
247 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
248 test characters of any code value, but the characters that PCRE recognizes as
249 digits, spaces, or word characters remain the same set as before, all with
250 values less than 256. This remains true even when PCRE includes Unicode
251 property support, because to do otherwise would slow down PCRE in many common
252 cases. If you really want to test for a wider sense of, say, "digit", you
253 must use Unicode property tests such as \ep{Nd}.
254 .P
255 7. Similarly, characters that match the POSIX named character classes are all
256 low-valued characters.
257 .P
258 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
259 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
260 .P
261 9. Case-insensitive matching applies only to characters whose values are less
262 than 128, unless PCRE is built with Unicode property support. Even when Unicode
263 property support is available, PCRE still uses its own character tables when
264 checking the case of low-valued characters, so as not to degrade performance.
265 The Unicode property information is used only for characters with higher
266 values. Even when Unicode property support is available, PCRE supports
267 case-insensitive matching only when there is a one-to-one mapping between a
268 letter's cases. There are a small number of many-to-one mappings in Unicode;
269 these are not supported by PCRE.
270 .
271 .
273 .rs
274 .sp
275 .nf
276 Philip Hazel
277 University Computing Service
278 Cambridge CB2 3QH, England.
279 .fi
280 .P
281 Putting an actual email address here seems to have been a spam magnet, so I've
282 taken it away. If you want to email me, use my two initials, followed by the
283 two digits 10, at the domain cam.ac.uk.
284 .
285 .
287 .rs
288 .sp
289 .nf
290 Last updated: 09 August 2007
291 Copyright (c) 1997-2007 University of Cambridge.
292 .fi


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