/[pcre]/code/trunk/doc/html/pcreunicode.html
ViewVC logotype

Contents of /code/trunk/doc/html/pcreunicode.html

Parent Directory Parent Directory | Revision Log Revision Log


Revision 903 - (show annotations)
Sat Jan 21 16:37:17 2012 UTC (7 years, 7 months ago) by ph10
File MIME type: text/html
File size: 10234 byte(s)
Source file tidies for 8.30-RC1 release; fix Makefile.am bugs for building 
symbolic links to man pages.
1 <html>
2 <head>
3 <title>pcreunicode specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcreunicode man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <br><b>
16 UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT
17 </b><br>
18 <P>
19 From Release 8.30, in addition to its previous UTF-8 support, PCRE also
20 supports UTF-16 by means of a separate 16-bit library. This can be built as
21 well as, or instead of, the 8-bit library.
22 </P>
23 <br><b>
24 UTF-8 SUPPORT
25 </b><br>
26 <P>
27 In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF
28 support, and, in addition, you must call
29 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
30 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
31 (*UTF8). When either of these is the case, both the pattern and any subject
32 strings that are matched against it are treated as UTF-8 strings instead of
33 strings of 1-byte characters.
34 </P>
35 <br><b>
36 UTF-16 SUPPORT
37 </b><br>
38 <P>
39 In order process UTF-16 strings, you must build PCRE's 16-bit library with UTF
40 support, and, in addition, you must call
41 <a href="pcre_compile.html"><b>pcre16_compile()</b></a>
42 with the PCRE_UTF16 option flag, or the pattern must start with the sequence
43 (*UTF16). When either of these is the case, both the pattern and any subject
44 strings that are matched against it are treated as UTF-16 strings instead of
45 strings of 16-bit characters.
46 </P>
47 <br><b>
48 UTF SUPPORT OVERHEAD
49 </b><br>
50 <P>
51 If you compile PCRE with UTF support, but do not use it at run time, the
52 library will be a bit bigger, but the additional run time overhead is limited
53 to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.
54 </P>
55 <br><b>
56 UNICODE PROPERTY SUPPORT
57 </b><br>
58 <P>
59 If PCRE is built with Unicode character property support (which implies UTF
60 support), the escape sequences \p{..}, \P{..}, and \X can be used.
61 The available properties that can be tested are limited to the general
62 category properties such as Lu for an upper case letter or Nd for a decimal
63 number, the Unicode script names such as Arabic or Han, and the derived
64 properties Any and L&. A full list is given in the
65 <a href="pcrepattern.html"><b>pcrepattern</b></a>
66 documentation. Only the short names for properties are supported. For example,
67 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
68 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
69 compatibility with Perl 5.6. PCRE does not support this.
70 <a name="utf8strings"></a></P>
71 <br><b>
72 Validity of UTF-8 strings
73 </b><br>
74 <P>
75 When you set the PCRE_UTF8 flag, the byte strings passed as patterns and
76 subjects are (by default) checked for validity on entry to the relevant
77 functions. From release 7.3 of PCRE, the check is according the rules of RFC
78 3629, which are themselves derived from the Unicode specification. Earlier
79 releases of PCRE followed the rules of RFC 2279, which allows the full range of
80 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the
81 range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.
82 </P>
83 <P>
84 The excluded code points are the "Surrogate Area" of Unicode. They are reserved
85 for use by UTF-16, where they are used in pairs to encode codepoints with
86 values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
87 are available independently in the UTF-8 encoding. (In other words, the whole
88 surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
89 </P>
90 <P>
91 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
92 compile time, the only additional information is the offset to the first byte
93 of the failing character. The runtime functions <b>pcre_exec()</b> and
94 <b>pcre_dfa_exec()</b> also pass back this information, as well as a more
95 detailed reason code if the caller has provided memory in which to do this.
96 </P>
97 <P>
98 In some situations, you may already know that your strings are valid, and
99 therefore want to skip these checks in order to improve performance. If you set
100 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
101 the pattern or subject it is given (respectively) contains only valid UTF-8
102 codes. In this case, it does not diagnose an invalid UTF-8 string.
103 </P>
104 <P>
105 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
106 happens depends on why the string is invalid. If the string conforms to the
107 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
108 in the range 0 to 0x7FFFFFFF by <b>pcre_dfa_exec()</b> and the interpreted
109 version of <b>pcre_exec()</b>. In other words, apart from the initial validity
110 test, these functions (when in UTF-8 mode) handle strings according to the more
111 liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
112 <b>pcre_exec()</b> supports only RFC 3629. If you are using JIT optimization, or
113 if the string does not even conform to RFC 2279, the result is undefined. Your
114 program may crash.
115 </P>
116 <P>
117 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
118 encoded in a UTF-8-like manner as per the old RFC, you can set
119 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
120 situation, you will have to apply your own validity check, and avoid the use of
121 JIT optimization.
122 <a name="utf16strings"></a></P>
123 <br><b>
124 Validity of UTF-16 strings
125 </b><br>
126 <P>
127 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are
128 passed as patterns and subjects are (by default) checked for validity on entry
129 to the relevant functions. Values other than those in the surrogate range
130 U+D800 to U+DFFF are independent code points. Values in the surrogate range
131 must be used in pairs in the correct manner.
132 </P>
133 <P>
134 If an invalid UTF-16 string is passed to PCRE, an error return is given. At
135 compile time, the only additional information is the offset to the first data
136 unit of the failing character. The runtime functions <b>pcre16_exec()</b> and
137 <b>pcre16_dfa_exec()</b> also pass back this information, as well as a more
138 detailed reason code if the caller has provided memory in which to do this.
139 </P>
140 <P>
141 In some situations, you may already know that your strings are valid, and
142 therefore want to skip these checks in order to improve performance. If you set
143 the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that
144 the pattern or subject it is given (respectively) contains only valid UTF-16
145 sequences. In this case, it does not diagnose an invalid UTF-16 string.
146 </P>
147 <br><b>
148 General comments about UTF modes
149 </b><br>
150 <P>
151 1. Codepoints less than 256 can be specified by either braced or unbraced
152 hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values
153 have to use braced sequences.
154 </P>
155 <P>
156 2. Octal numbers up to \777 are recognized, and in UTF-8 mode, they match
157 two-byte characters for values greater than \177.
158 </P>
159 <P>
160 3. Repeat quantifiers apply to complete UTF characters, not to individual
161 data units, for example: \x{100}{3}.
162 </P>
163 <P>
164 4. The dot metacharacter matches one UTF character instead of a single data
165 unit.
166 </P>
167 <P>
168 5. The escape sequence \C can be used to match a single byte in UTF-8 mode, or
169 a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange
170 effects because it breaks up multi-unit characters (see the description of \C
171 in the
172 <a href="pcrepattern.html"><b>pcrepattern</b></a>
173 documentation). The use of \C is not supported in the alternative matching
174 function <b>pcre[16]_dfa_exec()</b>, nor is it supported in UTF mode by the JIT
175 optimization of <b>pcre[16]_exec()</b>. If JIT optimization is requested for a
176 UTF pattern that contains \C, it will not succeed, and so the matching will
177 be carried out by the normal interpretive function.
178 </P>
179 <P>
180 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
181 test characters of any code value, but, by default, the characters that PCRE
182 recognizes as digits, spaces, or word characters remain the same set as in
183 non-UTF mode, all with values less than 256. This remains true even when PCRE
184 is built to include Unicode property support, because to do otherwise would
185 slow down PCRE in many common cases. Note in particular that this applies to
186 \b and \B, because they are defined in terms of \w and \W. If you really
187 want to test for a wider sense of, say, "digit", you can use explicit Unicode
188 property tests such as \p{Nd}. Alternatively, if you set the PCRE_UCP option,
189 the way that the character escapes work is changed so that Unicode properties
190 are used to determine which characters match. There are more details in the
191 section on
192 <a href="pcrepattern.html#genericchartypes">generic character types</a>
193 in the
194 <a href="pcrepattern.html"><b>pcrepattern</b></a>
195 documentation.
196 </P>
197 <P>
198 7. Similarly, characters that match the POSIX named character classes are all
199 low-valued characters, unless the PCRE_UCP option is set.
200 </P>
201 <P>
202 8. However, the horizontal and vertical whitespace matching escapes (\h, \H,
203 \v, and \V) do match all the appropriate Unicode characters, whether or not
204 PCRE_UCP is set.
205 </P>
206 <P>
207 9. Case-insensitive matching applies only to characters whose values are less
208 than 128, unless PCRE is built with Unicode property support. Even when Unicode
209 property support is available, PCRE still uses its own character tables when
210 checking the case of low-valued characters, so as not to degrade performance.
211 The Unicode property information is used only for characters with higher
212 values. Furthermore, PCRE supports case-insensitive matching only when there is
213 a one-to-one mapping between a letter's cases. There are a small number of
214 many-to-one mappings in Unicode; these are not supported by PCRE.
215 </P>
216 <br><b>
217 AUTHOR
218 </b><br>
219 <P>
220 Philip Hazel
221 <br>
222 University Computing Service
223 <br>
224 Cambridge CB2 3QH, England.
225 <br>
226 </P>
227 <br><b>
228 REVISION
229 </b><br>
230 <P>
231 Last updated: 13 January 2012
232 <br>
233 Copyright &copy; 1997-2012 University of Cambridge.
234 <br>
235 <p>
236 Return to the <a href="index.html">PCRE index page</a>.
237 </p>

  ViewVC Help
Powered by ViewVC 1.1.5