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PCREUNICODE(3)                                                  PCREUNICODE(3)



NAME
       PCRE - Perl-compatible regular expressions

UTF-8 AND UNICODE PROPERTY SUPPORT

       In order process UTF-8 strings, you must build PCRE to include UTF-8 support in the code, 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. PCRE does not support any other  formats  (in
       particular, it does not support UTF-16).

       If  you  compile  PCRE with UTF-8 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 flag occasionally, so should not  be  very
       big.

       If  PCRE  is  built with Unicode character property support (which implies UTF-8 support), the escape sequences
       \p{..}, \P{..}, and \X are supported.  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  documen-
       tation.  Only  the short names for properties are supported. For example, \p{L} matches a letter. Its Perl syn-
       onym, \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 strings passed as patterns and subjects are (by default) checked for
       validity on entry to the relevant functions. 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 val-
       ues in the range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.

       The excluded code points are the "Low Surrogate Area" of Unicode, of which the Unicode Standard says this: "The
       Low Surrogate Area does not contain any  character  assignments,  consequently  no  character  code  charts  or
       namelists  are  provided  for  this  area. Surrogates are reserved for use with UTF-16 and then must be used in
       pairs." The code points that are encoded by UTF-16 pairs are available as independent code points 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 runtime functions pcre_exec() and
       pcre_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has  pro-
       vided 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_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 situa-
       tion, you will have to apply your own validity check, and avoid the use of JIT optimization.

   General comments about UTF-8 mode

       1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte UTF-8 character if  the  value  is
       greater than 127.

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

       3. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \x{100}{3}.

       4. The dot metacharacter matches one UTF-8 character instead of a single byte.

       5.  The  escape  sequence  \C  can  be  used to match a single byte in UTF-8 mode, but its use can lead to some
       strange effects because it breaks up multibyte characters (see the description of \C in the  pcrepattern  docu-
       mentation). The use of \C is not supported in the alternative matching function pcre_dfa_exec(), nor is it sup-
       ported in UTF-8 mode by the JIT optimization of pcre_exec(). If JIT optimization is requested for a UTF-8  pat-
       tern  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
       before, 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  whitespace 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 charac-
       ter 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-insensi-
       tive 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: 19 October 2011
       Copyright (c) 1997-2011 University of Cambridge.



                                                                PCREUNICODE(3)