Hi, I would recomment on using the recursive parser toolkit from Damian Conway. Although you should be able to do it with some regexes and a stack-machine also. A single RE will not work though, as this problem is non trivial. I read in one of the replies that you'll need a lexer and a parser ( execellent reply! ) but they can get very simple. The lexer can be a simple list of regexes and the parser can be a simple array in which you pop, push stuff ( stack ). A good pointer to solve this problem can be:
http://www.yapc.org/Europe/2001/proceedings/68/index.html
and
http://www.samag.com/documents/s=1288/sam03040010/

Hope this helps a bit ...

J.