Never used ply, but why not regex? Regex cannot easily match recursive structures, and can't be easily composed. Pyparsing gives you more structure and ready building blocks. It actually can use regex as one of the term objects.
Ply tries to implement lex/yacc only, so it's far from a nice interface really...
Or specifically: it's trivial to match `specific_func(any_expression, {capture, these, values})`, but it's practically impossible with regex and painful to post-process with ply.
"I've noticed that when I get quoted in .sig files, it's never any of the actual clever things I say all the time. Usually it's something dumb." --Jamie Zawinski
