make parser recognize keys with non-ASCII characters

Hello,

I found out that the BibTeX parser was not able to correctly parse keys containing umlauts, e. g. "müller.2011", instead giving me the warning ```Lexer: unexpected token `ü'```. When I changed the relevant regular expressions to use `[:digit:]` instead of previous `\d` everything worked as expected, although I'm not exactly sure why.

Regards,
Christoph