[LawnGnome]

Happens in PHP only if you enable Unicode regex handling via the /u modifier and are running libpcre 8.10 or later (which corresponds to PHP 5.3.4 and later, assuming you're using the bundled libpcre): http://3v4l.org/QD3k0

[bodyfour]

If you're using pcre directly from C code, this is controlled by specifying the PCRE_UCP flag to pcre_compile(). By default, \d and friends only match ASCII characters even if the PCRE_UTF8 flag is set.