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Update UTF-8 validity check and documentation.

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 nigel 93 differences. (Certain features that appeared in Python and PCRE before they
10     appeared in Perl are also available using the Python syntax.)
11 nigel 75 .P
12 nigel 93 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 nigel 77 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 nigel 75 PCRE is written in C and released as a C library. A number of people have
28 nigel 77 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 nigel 75 .sp
37 nigel 63 .\" 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 nigel 75 .P
41 nigel 63 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 ph10 208 pages. There is a syntax summary in the
51     .\" HREF
52     \fBpcresyntax\fR
53     .\"
54     page.
55 nigel 75 .P
56 nigel 63 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 nigel 75 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 nigel 77 .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 nigel 83 "_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 nigel 75 .
77     .
79 nigel 63 .rs
80     .sp
81 nigel 75 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 nigel 63 pcre this document
88 ph10 153 pcre-config show PCRE installation configuration information
89 nigel 77 pcreapi details of PCRE's native C API
90 nigel 63 pcrebuild options for building PCRE
91     pcrecallout details of the callout feature
92     pcrecompat discussion of Perl compatibility
93 nigel 77 pcrecpp details of the C++ wrapper
94 nigel 75 pcregrep description of the \fBpcregrep\fP command
95 nigel 77 pcrematching discussion of the two matching algorithms
96 nigel 75 pcrepartial details of the partial matching facility
97     .\" JOIN
98 nigel 63 pcrepattern syntax and semantics of supported
99     regular expressions
100 ph10 208 pcresyntax quick syntax reference
101 nigel 63 pcreperform discussion of performance issues
102 nigel 77 pcreposix the POSIX-compatible C API
103 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
104 nigel 63 pcresample discussion of the sample program
105 nigel 91 pcrestack discussion of stack usage
106 nigel 75 pcretest description of the \fBpcretest\fP testing command
107     .sp
108 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
109 nigel 77 C library function, listing its arguments and results.
110 nigel 75 .
111     .
112 nigel 41 .SH LIMITATIONS
113 nigel 63 .rs
114     .sp
115 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
116     practice be relevant.
117 nigel 75 .P
118 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
122 nigel 63 distribution and the
123     .\" HREF
124 nigel 75 \fBpcrebuild\fP
125 nigel 63 .\"
126 nigel 75 documentation for details). In these cases the limit is substantially larger.
127 nigel 93 However, the speed of execution is slower.
128 nigel 75 .P
129 ph10 208 All values in repeating quantifiers must be less than 65536.
130 nigel 75 .P
131 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
132     no more than 65535 capturing subpatterns.
133 nigel 75 .P
134 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
135     maximum number of named subpatterns is 10000.
136 nigel 91 .P
137 nigel 41 The maximum length of a subject string is the largest positive number that an
138 nigel 77 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 nigel 91 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 ph10 211 .
148 nigel 63 .\" HTML <a name="utf8support"></a>
149 nigel 75 .
150     .
152 nigel 63 .rs
153     .sp
154 nigel 75 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 nigel 63 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 nigel 75 \fBpcre_compile()\fP
163 nigel 63 .\"
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 nigel 75 .P
168 nigel 49 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 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
171 nigel 75 .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 nigel 87 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 nigel 75 .\" HREF
179     \fBpcrepattern\fP
180     .\"
181 nigel 87 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 ph10 211 .
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 nigel 75 .P
199 ph10 211 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 nigel 75 .P
208 ph10 211 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 nigel 75 .P
216 ph10 211 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 ph10 209 .P
224 ph10 211 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 nigel 87 UTF-8 character if the value is greater than 127.
234 nigel 75 .P
235 ph10 211 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
236 nigel 91 characters for values greater than \e177.
237     .P
238 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
239 nigel 75 bytes, for example: \ex{100}{3}.
240     .P
241 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
242 nigel 75 .P
243 ph10 211 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
244 nigel 77 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 nigel 75 .P
247 ph10 211 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
248 nigel 63 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 nigel 75 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 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
256 nigel 75 low-valued characters.
257     .P
258 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
259 ph10 181 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
260     .P
261 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
262 nigel 75 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 nigel 87 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 nigel 75 .
271 ph10 99 .
272 nigel 41 .SH AUTHOR
273 nigel 63 .rs
274     .sp
275 ph10 99 .nf
276 nigel 77 Philip Hazel
277 ph10 99 University Computing Service
278 nigel 93 Cambridge CB2 3QH, England.
279 ph10 99 .fi
280 nigel 77 .P
281     Putting an actual email address here seems to have been a spam magnet, so I've
282 ph10 153 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 ph10 99 .
285     .
286     .SH REVISION
287     .rs
288 nigel 75 .sp
289 ph10 99 .nf
290 ph10 211 Last updated: 09 August 2007
291 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
292     .fi


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