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. 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 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 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_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 runtime 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.
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 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 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: 13 January 2012
Copyright (c) 1997-2012 University of Cambridge.
