pcreunicode


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 sup-
       port 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 rel-
       evant functions. The entire string is checked before any other process-
       ing  takes  place. From release 7.3 of PCRE, the check is according the
       rules of RFC 3629, which are themselves derived from the Unicode speci-
       fication.  Earlier  releases  of  PCRE  followed the rules of RFC 2279,
       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 perfor-
       mance, 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  con-
       forms 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 con-
       form 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 valid-
       ity 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  perfor-
       mance.  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 (respec-
       tively) contains only valid UTF-16 sequences. In this case, it does not
       diagnose an invalid UTF-16 string.

       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  opti-
       mization 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 sec-
       tion 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 map-
       pings 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.
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