What is the PCRE pattern, in PHP, for characters with hex code from \x00 to \xFF?
To match a specific Unicode code point, use \uFFFF where FFFF is the hexadecimal number of the code point you want to match. You must always specify 4 hexadecimal digits. E.g. \u00E0 matches "à", but only when encoded as a single code point U+00E0.
Source: Regex Tutorial