/[pcre]/code/trunk/doc/pcreunicode.3
ViewVC logotype

Contents of /code/trunk/doc/pcreunicode.3

Parent Directory Parent Directory | Revision Log Revision Log


Revision 959 - (show annotations)
Sat Apr 14 16:16:58 2012 UTC (7 years, 4 months ago) by ph10
File size: 9800 byte(s)
Update documentation to clarify that UTF-8/16 checking is done on complete 
strings before any other processing.
1 .TH PCREUNICODE 3 "14 April 2012" "PCRE 8.30"
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT"
5 .rs
6 .sp
7 From Release 8.30, in addition to its previous UTF-8 support, PCRE also
8 supports UTF-16 by means of a separate 16-bit library. This can be built as
9 well as, or instead of, the 8-bit library.
10 .
11 .
12 .SH "UTF-8 SUPPORT"
13 .rs
14 .sp
15 In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF
16 support, and, in addition, you must call
17 .\" HREF
18 \fBpcre_compile()\fP
19 .\"
20 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
21 (*UTF8). When either of these is the case, both the pattern and any subject
22 strings that are matched against it are treated as UTF-8 strings instead of
23 strings of 1-byte characters.
24 .
25 .
26 .SH "UTF-16 SUPPORT"
27 .rs
28 .sp
29 In order process UTF-16 strings, you must build PCRE's 16-bit library with UTF
30 support, and, in addition, you must call
31 .\" HTML <a href="pcre_compile.html">
32 .\" </a>
33 \fBpcre16_compile()\fP
34 .\"
35 with the PCRE_UTF16 option flag, or the pattern must start with the sequence
36 (*UTF16). When either of these is the case, both the pattern and any subject
37 strings that are matched against it are treated as UTF-16 strings instead of
38 strings of 16-bit characters.
39 .
40 .
41 .SH "UTF SUPPORT OVERHEAD"
42 .rs
43 .sp
44 If you compile PCRE with UTF support, but do not use it at run time, the
45 library will be a bit bigger, but the additional run time overhead is limited
46 to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.
47 .
48 .
49 .SH "UNICODE PROPERTY SUPPORT"
50 .rs
51 .sp
52 If PCRE is built with Unicode character property support (which implies UTF
53 support), the escape sequences \ep{..}, \eP{..}, and \eX can be used.
54 The available properties that can be tested are limited to the general
55 category properties such as Lu for an upper case letter or Nd for a decimal
56 number, the Unicode script names such as Arabic or Han, and the derived
57 properties Any and L&. A full list is given in the
58 .\" HREF
59 \fBpcrepattern\fP
60 .\"
61 documentation. Only the short names for properties are supported. For example,
62 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
63 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
64 compatibility with Perl 5.6. PCRE does not support this.
65 .
66 .
67 .\" HTML <a name="utf8strings"></a>
68 .SS "Validity of UTF-8 strings"
69 .rs
70 .sp
71 When you set the PCRE_UTF8 flag, the byte strings passed as patterns and
72 subjects are (by default) checked for validity on entry to the relevant
73 functions. The entire string is checked before any other processing takes
74 place. From release 7.3 of PCRE, the check is according the rules of RFC 3629,
75 which are themselves derived from the Unicode specification. Earlier releases
76 of PCRE followed the rules of RFC 2279, which allows the full range of 31-bit
77 values (0 to 0x7FFFFFFF). The current check allows only values in the range U+0
78 to U+10FFFF, excluding U+D800 to U+DFFF.
79 .P
80 The excluded code points are the "Surrogate Area" of Unicode. They are reserved
81 for use by UTF-16, where they are used in pairs to encode codepoints with
82 values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
83 are available independently in the UTF-8 encoding. (In other words, the whole
84 surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
85 .P
86 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
87 compile time, the only additional information is the offset to the first byte
88 of the failing character. The runtime functions \fBpcre_exec()\fP and
89 \fBpcre_dfa_exec()\fP also pass back this information, as well as a more
90 detailed reason code if the caller has provided memory in which to do this.
91 .P
92 In some situations, you may already know that your strings are valid, and
93 therefore want to skip these checks in order to improve performance, for
94 example in the case of a long subject string that is being scanned repeatedly
95 with different patterns. If you set the PCRE_NO_UTF8_CHECK flag at compile time
96 or at run time, PCRE assumes that the pattern or subject it is given
97 (respectively) contains only valid UTF-8 codes. In this case, it does not
98 diagnose an invalid UTF-8 string.
99 .P
100 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
101 happens depends on why the string is invalid. If the string conforms to the
102 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
103 in the range 0 to 0x7FFFFFFF by \fBpcre_dfa_exec()\fP and the interpreted
104 version of \fBpcre_exec()\fP. In other words, apart from the initial validity
105 test, these functions (when in UTF-8 mode) handle strings according to the more
106 liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
107 \fBpcre_exec()\fP supports only RFC 3629. If you are using JIT optimization, or
108 if the string does not even conform to RFC 2279, the result is undefined. Your
109 program may crash.
110 .P
111 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
112 encoded in a UTF-8-like manner as per the old RFC, you can set
113 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
114 situation, you will have to apply your own validity check, and avoid the use of
115 JIT optimization.
116 .
117 .
118 .\" HTML <a name="utf16strings"></a>
119 .SS "Validity of UTF-16 strings"
120 .rs
121 .sp
122 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are
123 passed as patterns and subjects are (by default) checked for validity on entry
124 to the relevant functions. Values other than those in the surrogate range
125 U+D800 to U+DFFF are independent code points. Values in the surrogate range
126 must be used in pairs in the correct manner.
127 .P
128 If an invalid UTF-16 string is passed to PCRE, an error return is given. At
129 compile time, the only additional information is the offset to the first data
130 unit of the failing character. The runtime functions \fBpcre16_exec()\fP and
131 \fBpcre16_dfa_exec()\fP also pass back this information, as well as a more
132 detailed reason code if the caller has provided memory in which to do this.
133 .P
134 In some situations, you may already know that your strings are valid, and
135 therefore want to skip these checks in order to improve performance. If you set
136 the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that
137 the pattern or subject it is given (respectively) contains only valid UTF-16
138 sequences. In this case, it does not diagnose an invalid UTF-16 string.
139 .
140 .
141 .SS "General comments about UTF modes"
142 .rs
143 .sp
144 1. Codepoints less than 256 can be specified by either braced or unbraced
145 hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger values
146 have to use braced sequences.
147 .P
148 2. Octal numbers up to \e777 are recognized, and in UTF-8 mode, they match
149 two-byte characters for values greater than \e177.
150 .P
151 3. Repeat quantifiers apply to complete UTF characters, not to individual
152 data units, for example: \ex{100}{3}.
153 .P
154 4. The dot metacharacter matches one UTF character instead of a single data
155 unit.
156 .P
157 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode, or
158 a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange
159 effects because it breaks up multi-unit characters (see the description of \eC
160 in the
161 .\" HREF
162 \fBpcrepattern\fP
163 .\"
164 documentation). The use of \eC is not supported in the alternative matching
165 function \fBpcre[16]_dfa_exec()\fP, nor is it supported in UTF mode by the JIT
166 optimization of \fBpcre[16]_exec()\fP. If JIT optimization is requested for a
167 UTF pattern that contains \eC, it will not succeed, and so the matching will
168 be carried out by the normal interpretive function.
169 .P
170 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
171 test characters of any code value, but, by default, the characters that PCRE
172 recognizes as digits, spaces, or word characters remain the same set as in
173 non-UTF mode, all with values less than 256. This remains true even when PCRE
174 is built to include Unicode property support, because to do otherwise would
175 slow down PCRE in many common cases. Note in particular that this applies to
176 \eb and \eB, because they are defined in terms of \ew and \eW. If you really
177 want to test for a wider sense of, say, "digit", you can use explicit Unicode
178 property tests such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option,
179 the way that the character escapes work is changed so that Unicode properties
180 are used to determine which characters match. There are more details in the
181 section on
182 .\" HTML <a href="pcrepattern.html#genericchartypes">
183 .\" </a>
184 generic character types
185 .\"
186 in the
187 .\" HREF
188 \fBpcrepattern\fP
189 .\"
190 documentation.
191 .P
192 7. Similarly, characters that match the POSIX named character classes are all
193 low-valued characters, unless the PCRE_UCP option is set.
194 .P
195 8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
196 \ev, and \eV) do match all the appropriate Unicode characters, whether or not
197 PCRE_UCP is set.
198 .P
199 9. Case-insensitive matching applies only to characters whose values are less
200 than 128, unless PCRE is built with Unicode property support. Even when Unicode
201 property support is available, PCRE still uses its own character tables when
202 checking the case of low-valued characters, so as not to degrade performance.
203 The Unicode property information is used only for characters with higher
204 values. Furthermore, PCRE supports case-insensitive matching only when there is
205 a one-to-one mapping between a letter's cases. There are a small number of
206 many-to-one mappings in Unicode; these are not supported by PCRE.
207 .
208 .
209 .SH AUTHOR
210 .rs
211 .sp
212 .nf
213 Philip Hazel
214 University Computing Service
215 Cambridge CB2 3QH, England.
216 .fi
217 .
218 .
219 .SH REVISION
220 .rs
221 .sp
222 .nf
223 Last updated: 14 April 2012
224 Copyright (c) 1997-2012 University of Cambridge.
225 .fi

  ViewVC Help
Powered by ViewVC 1.1.5