Provided by: libpcre3-dev_8.38-3.1_amd64 
NAME
PCRE - Perl-compatible regular expressions
UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT
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
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
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
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
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
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 27 February 2013
Copyright (c) 1997-2013 University of Cambridge.