Are the PHP preg_functions multibyte safe?
pcre supports utf8 out of the box, see documentation for the 'u' modifier.
Illustration (\xC3\xA4 is the utf8 encoding for the german letter "ä")
echo preg_replace('~\w~', '@', "a\xC3\xA4b");
this echoes "@@¤@" because "\xC3" and "\xA4" were treated as distinct symbols
echo preg_replace('~\w~u', '@', "a\xC3\xA4b");
(note the 'u') prints "@@@" because "\xC3\xA4" were treated as a single letter.
PCRE can support UTF-8 and other Unicode encodings, but it has to be specified at compile time. From the man page for PCRE 8.0:
The current implementation of PCRE corresponds approximately with Perl 5.10, including support for UTF-8 encoded strings and Unicode general category properties. However, UTF-8 and Unicode support has to be explicitly enabled; it is not the default. The Unicode tables correspond to Unicode release 5.1.
PHP currently uses PCRE 7.9; your system might have an older version.
Taking a look at the PCRE lib that comes with PHP 5.2, it appears that it's configured to support Unicode properties and UTF-8. Same for the 5.3 branch.