PCREUNICODE

Section: C Library Functions (3)
Updated: 14 April 2012
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NAME

PCRE - Perl-compatible regular expressions  

UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT

From Release 8.30, in addition to its previous UTF-8 support, PCRE also supports UTF-16 by means of a separate 16-bit library. This can be built as well as, or instead of, the 8-bit library.  

UTF-8 SUPPORT

In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF support, and, in addition, you must call pcre_compile() with the PCRE_UTF8 option flag, or the pattern must start with the sequence (*UTF8). When either of these is the case, both the pattern and any subject strings that are matched against it are treated as UTF-8 strings instead of strings of 1-byte characters.  

UTF-16 SUPPORT

In order process UTF-16 strings, you must build PCRE's 16-bit library with UTF support, and, in addition, you must call pcre16_compile() with the PCRE_UTF16 option flag, or the pattern must start with the sequence (*UTF16). When either of these is the case, both the pattern and any subject strings that are matched against it are treated as UTF-16 strings instead of strings of 16-bit characters.  

UTF SUPPORT OVERHEAD

If you compile PCRE with UTF support, but do not use it at run time, the library will be a bit bigger, but the additional run time overhead is limited to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.  

UNICODE PROPERTY SUPPORT

If PCRE is built with Unicode character property support (which implies UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. The available properties that can be tested are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal number, the Unicode script names such as Arabic or Han, and the derived properties Any and L&. A full list is given in the pcrepattern documentation. Only the short names for properties are supported. For example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this.  

Validity of UTF-8 strings

When you set the PCRE_UTF8 flag, the byte strings passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. The entire string is checked before any other processing takes place. From release 7.3 of PCRE, the check is according the rules of RFC 3629, which are themselves derived from the Unicode specification. Earlier releases of PCRE followed the rules of RFC 2279, which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.

The excluded code points are the "Surrogate Area" of Unicode. They are reserved for use by UTF-16, where they are used in pairs to encode codepoints with values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs are available independently in the UTF-8 encoding. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)

If an invalid UTF-8 string is passed to PCRE, an error return is given. At compile time, the only additional information is the offset to the first byte of the failing character. The run-time functions pcre_exec() and pcre_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has provided memory in which to do this.

In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve performance, for example in the case of a long subject string that is being scanned repeatedly with different patterns. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respectively) contains only valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8 string.

If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what happens depends on why the string is invalid. If the string conforms to the "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec() and the interpreted version of pcre_exec(). In other words, apart from the initial validity test, these functions (when in UTF-8 mode) handle strings according to the more liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629. If you are using JIT optimization, or if the string does not even conform to RFC 2279, the result is undefined. Your program may crash.

If you want to process strings of values in the full range 0 to 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this situation, you will have to apply your own validity check, and avoid the use of JIT optimization.  

Validity of UTF-16 strings

When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. Values other than those in the surrogate range U+D800 to U+DFFF are independent code points. Values in the surrogate range must be used in pairs in the correct manner.

If an invalid UTF-16 string is passed to PCRE, an error return is given. At compile time, the only additional information is the offset to the first data unit of the failing character. The run-time functions pcre16_exec() and pcre16_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has provided memory in which to do this.

In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve performance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respectively) contains only valid UTF-16 sequences. In this case, it does not diagnose an invalid UTF-16 string.  

General comments about UTF modes

1. Codepoints less than 256 can be specified by either braced or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values have to use braced sequences.

2. Octal numbers up to \777 are recognized, and in UTF-8 mode, they match two-byte characters for values greater than \177.

3. Repeat quantifiers apply to complete UTF characters, not to individual data units, for example: \x{100}{3}.

4. The dot metacharacter matches one UTF character instead of a single data unit.

5. The escape sequence \C can be used to match a single byte in UTF-8 mode, or a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange effects because it breaks up multi-unit characters (see the description of \C in the pcrepattern documentation). The use of \C is not supported in the alternative matching function pcre[16]_dfa_exec(), nor is it supported in UTF mode by the JIT optimization of pcre[16]_exec(). If JIT optimization is requested for a UTF pattern that contains \C, it will not succeed, and so the matching will be carried out by the normal interpretive function.

6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but, by default, the characters that PCRE recognizes as digits, spaces, or word characters remain the same set as in non-UTF mode, all with values less than 256. This remains true even when PCRE is built to include Unicode property support, because to do otherwise would slow down PCRE in many common cases. Note in particular that this applies to \b and \B, because they are defined in terms of \w and \W. If you really want to test for a wider sense of, say, "digit", you can use explicit Unicode property tests such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the character escapes work is changed so that Unicode properties are used to determine which characters match. There are more details in the section on generic character types in the pcrepattern documentation.

7. Similarly, characters that match the POSIX named character classes are all low-valued characters, unless the PCRE_UCP option is set.

8. However, the horizontal and vertical white space matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode characters, whether or not PCRE_UCP is set.

9. Case-insensitive matching applies only to characters whose values are less than 128, unless PCRE is built with Unicode property support. Even when Unicode property support is available, PCRE still uses its own character tables when checking the case of low-valued characters, so as not to degrade performance. The Unicode property information is used only for characters with higher values. Furthermore, PCRE supports case-insensitive matching only when there is a one-to-one mapping between a letter's cases. There are a small number of many-to-one mappings in Unicode; these are not supported by PCRE.  

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
 

REVISION

Last updated: 14 April 2012
Copyright (c) 1997-2012 University of Cambridge.


 

Index

NAME
UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT
UTF-8 SUPPORT
UTF-16 SUPPORT
UTF SUPPORT OVERHEAD
UNICODE PROPERTY SUPPORT
Validity of UTF-8 strings
Validity of UTF-16 strings
General comments about UTF modes
AUTHOR
REVISION

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Time: 05:22:40 GMT, January 20, 2018