Sorry, that was bad advice from me regarding bygroups() and words(). I forgot that words() introduces a capturing group. So the (words(..., prefix=r'(', suffix=r')( )'), bygroups(X, Y)) translated to (r'((...)( ))', bygroups(X, Y)), which doesn't give the expected result (the first 2 capturing groups are the outer one and the first inner one). I've fixed it.