support named character classes

[christianparpart]

See https://en.wikipedia.org/wiki/Regular_expression#Character_classes for a list of standard named character classes.

But especially those that are also in flex:

[:alnum:] [:alpha:] [:blank:]
[:cntrl:] [:digit:] [:graph:]
[:lower:] [:print:] [:punct:]
[:space:] [:upper:] [:xdigit:]