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.

PCRE 8.30                                         14 April 2012                                   PCREUNICODE(3)