pcreunicode(3) — Linux manual page

NAME | UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT | UTF-8 SUPPORT | UTF-16 AND UTF-32 SUPPORT | UTF SUPPORT OVERHEAD | UNICODE PROPERTY SUPPORT | AUTHOR | REVISION | COLOPHON

PCREUNICODE(3)          Library Functions Manual          PCREUNICODE(3)

NAME         top

       PCRE - Perl-compatible regular expressions

UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT         top


       As well as UTF-8 support, PCRE also supports UTF-16 (from release
       8.30) and UTF-32 (from release 8.32), by means of two additional
       libraries. They can be built as well as, or instead of, the 8-bit
       library.

UTF-8 SUPPORT         top


       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) or (*UTF). 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 individual 1-byte characters.

UTF-16 AND UTF-32 SUPPORT         top


       In order process UTF-16 or UTF-32 strings, you must build PCRE's
       16-bit or 32-bit library with UTF support, and, in addition, you
       must call pcre16_compile() or pcre32_compile() with the
       PCRE_UTF16 or PCRE_UTF32 option flag, as appropriate.
       Alternatively, the pattern must start with the sequence (*UTF16),
       (*UTF32), as appropriate, or (*UTF), which can be used with
       either library. When UTF mode is set, both the pattern and any
       subject strings that are matched against it are treated as UTF-16
       or UTF-32 strings instead of strings of individual 16-bit or
       32-bit characters.

UTF SUPPORT OVERHEAD         top


       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_UTF[8|16|32] flag
       occasionally, so should not be very big.

UNICODE PROPERTY SUPPORT         top


       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&. Full lists is given in the pcrepattern and pcresyntax
       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 the
       surrogate area. (From release 8.33 the so-called "non-character"
       code points are no longer excluded because Unicode corrigendum #9
       makes it clear that they should not be.)

       Characters in the "Surrogate Area" of Unicode 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 and
       UTF-32 encodings. (In other words, the whole surrogate thing is a
       fudge for UTF-16 which unfortunately messes up UTF-8 and UTF-32.)

       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.  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.

       Note that passing PCRE_NO_UTF8_CHECK to pcre_compile() just
       disables the check for the pattern; it does not also apply to
       subject strings. If you want to disable the check for a subject
       string you must pass this option to pcre_exec() or
       pcre_dfa_exec().

       If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is
       set, the result is undefined and your program may crash.

   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.  However, if an invalid string is passed, the result is
       undefined.

   Validity of UTF-32 strings

       When you set the PCRE_UTF32 flag, the strings of 32-bit data
       units that are passed as patterns and subjects are (by default)
       checked for validity on entry to the relevant functions.  This
       check allows only values in the range U+0 to U+10FFFF, excluding
       the surrogate area U+D800 to U+DFFF.

       If an invalid UTF-32 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 pcre32_exec() and pcre32_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_UTF32_CHECK flag at
       compile time or at run time, PCRE assumes that the pattern or
       subject it is given (respectively) contains only valid UTF-32
       sequences. In this case, it does not diagnose an invalid UTF-32
       string.  However, if an invalid string is passed, the result is
       undefined.

   General comments about UTF modes

       1. Codepoints less than 256 can be specified in patterns 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, or a
       single 32-bit data unit in UTF-32 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|32]_dfa_exec(), nor is it supported in UTF mode by the
       JIT optimization of pcre[16|32]_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. A few Unicode characters such as Greek sigma
       have more than two codepoints that are case-equivalent. Up to and
       including PCRE release 8.31, only one-to-one case mappings were
       supported, but later releases (with Unicode property support) do
       treat as case-equivalent all versions of characters such as Greek
       sigma.

AUTHOR         top


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

REVISION         top


       Last updated: 27 February 2013
       Copyright (c) 1997-2013 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.44.tar.gz fetched from
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2021-04-01.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

PCRE 8.33                   27 February 2013              PCREUNICODE(3)