Allowed characters for CSS identifiers

Question

What are the (full) valid / allowed charset characters for CSS identifiers id and class?

Is there a regular expression tha

Answer 1

For anyone looking for something a little more turn-key. The full expression, replaced and all, from @BalusC's answer is:

/-?([_a-z]|[\240-\377]|([0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|[^\r\n\f0-9a-f]))([_a-z0-9-]|[\240-\377]|([0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|[^\r\n\f0-9a-f]))*/

And using DEFINE, which I find a little more readable:

/(?(DEFINE)
    (?P        [0-9a-f]                             )
    (?P  (?&h){1,6}(\r\n|[ \t\r\n\f])?        )
    (?P   ((?&unicode)|[^\r\n\f0-9a-f])*       )
    (?P [\240-\377]                          )
    (?P   ([_a-z0-9-]|(?&nonascii)|(?&escape)) )
    (?P  ([_a-z]|(?&nonascii)|(?&escape))     )
    (?P    -?(?&nmstart)(?&nmchar)*             )
) (?:
    (?&ident)
)/x

Incidentally, the original regular expression (and @human's contribution) had a few rogue escape characters that allow [ in the name.

Also, it should be noted that the raw regex without, DEFINE, runs about 2x as fast as the DEFINE expression, taking only ~23 steps to identify a single unicode character, while the later takes ~40.