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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 |
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PCREUNICODE(3) Library Functions Manual PCREUNICODE(3)
PCRE - Perl-compatible regular expressions
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.
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.
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.
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.
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.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 27 February 2013
Copyright (c) 1997-2013 University of Cambridge.
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-8.45.tar.gz fetched from
⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
2021-08-27. 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)
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