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Documentation for JIT support.
3 PCRE - Perl-compatible regular expressions
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7 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
8 the code, and, in addition, you must call
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10 \fBpcre_compile()\fP
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12 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
13 (*UTF8). When either of these is the case, both the pattern and any subject
14 strings that are matched against it are treated as UTF-8 strings instead of
15 strings of 1-byte characters. PCRE does not support any other formats (in
16 particular, it does not support UTF-16).
17 .P
18 If you compile PCRE with UTF-8 support, but do not use it at run time, the
19 library will be a bit bigger, but the additional run time overhead is limited
20 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
21 .P
22 If PCRE is built with Unicode character property support (which implies UTF-8
23 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
24 The available properties that can be tested are limited to the general
25 category properties such as Lu for an upper case letter or Nd for a decimal
26 number, the Unicode script names such as Arabic or Han, and the derived
27 properties Any and L&. A full list is given in the
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29 \fBpcrepattern\fP
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31 documentation. Only the short names for properties are supported. For example,
32 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
33 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
34 compatibility with Perl 5.6. PCRE does not support this.
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37 .\" HTML <a name="utf8strings"></a>
38 .SS "Validity of UTF-8 strings"
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41 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
42 are (by default) checked for validity on entry to the relevant functions. From
43 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
44 themselves derived from the Unicode specification. Earlier releases of PCRE
45 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
46 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
47 U+10FFFF, excluding U+D800 to U+DFFF.
48 .P
49 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
50 Unicode Standard says this: "The Low Surrogate Area does not contain any
51 character assignments, consequently no character code charts or namelists are
52 provided for this area. Surrogates are reserved for use with UTF-16 and then
53 must be used in pairs." The code points that are encoded by UTF-16 pairs are
54 available as independent code points in the UTF-8 encoding. (In other words,
55 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
56 UTF-8.)
57 .P
58 If an invalid UTF-8 string is passed to PCRE, an error return is given. At
59 compile time, the only additional information is the offset to the first byte
60 of the failing character. The runtime functions \fBpcre_exec()\fP and
61 \fBpcre_dfa_exec()\fP also pass back this information, as well as a more
62 detailed reason code if the caller has provided memory in which to do this.
63 .P
64 In some situations, you may already know that your strings are valid, and
65 therefore want to skip these checks in order to improve performance. If you set
66 the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
67 the pattern or subject it is given (respectively) contains only valid UTF-8
68 codes. In this case, it does not diagnose an invalid UTF-8 string.
69 .P
70 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
71 happens depends on why the string is invalid. If the string conforms to the
72 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
73 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
74 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
75 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
76 the result is undefined. Your program may crash.
77 .P
78 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
79 encoded in a UTF-8-like manner as per the old RFC, you can set
80 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
81 situation, you will have to apply your own validity check.
82 .
83 .
84 .SS "General comments about UTF-8 mode"
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87 1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
88 UTF-8 character if the value is greater than 127.
89 .P
90 2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
91 characters for values greater than \e177.
92 .P
93 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
94 bytes, for example: \ex{100}{3}.
95 .P
96 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
97 .P
98 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
99 but its use can lead to some strange effects. This facility is not available in
100 the alternative matching function, \fBpcre_dfa_exec()\fP.
101 .P
102 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
103 test characters of any code value, but, by default, the characters that PCRE
104 recognizes as digits, spaces, or word characters remain the same set as before,
105 all with values less than 256. This remains true even when PCRE is built to
106 include Unicode property support, because to do otherwise would slow down PCRE
107 in many common cases. Note in particular that this applies to \eb and \eB,
108 because they are defined in terms of \ew and \eW. If you really want to test
109 for a wider sense of, say, "digit", you can use explicit Unicode property tests
110 such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
111 the character escapes work is changed so that Unicode properties are used to
112 determine which characters match. There are more details in the section on
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115 generic character types
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117 in the
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119 \fBpcrepattern\fP
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121 documentation.
122 .P
123 7. Similarly, characters that match the POSIX named character classes are all
124 low-valued characters, unless the PCRE_UCP option is set.
125 .P
126 8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
127 \ev, and \eV) do match all the appropriate Unicode characters, whether or not
128 PCRE_UCP is set.
129 .P
130 9. Case-insensitive matching applies only to characters whose values are less
131 than 128, unless PCRE is built with Unicode property support. Even when Unicode
132 property support is available, PCRE still uses its own character tables when
133 checking the case of low-valued characters, so as not to degrade performance.
134 The Unicode property information is used only for characters with higher
135 values. Furthermore, PCRE supports case-insensitive matching only when there is
136 a one-to-one mapping between a letter's cases. There are a small number of
137 many-to-one mappings in Unicode; these are not supported by PCRE.
138 .
139 .
141 .rs
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143 .nf
144 Philip Hazel
145 University Computing Service
146 Cambridge CB2 3QH, England.
147 .fi
148 .
149 .
151 .rs
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153 .nf
154 Last updated: 24 August 2011
155 Copyright (c) 1997-2011 University of Cambridge.
156 .fi

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