RegEx Parser written in Prolog

Question

I\'ve been banging my head against the wall on this homework problem for a few hours now. We have to parse a regular expression with Prolog. For the most part, the predicat

Answer 1

I don't have access to SWI Prolog right now, but here is a guess:

Try changing

re_match(star(Rx), List) :- append(List1, List2, List),
                            re_match(Rx, List1),
                            re_match(star(Rx), List2).

to

re_match(star(Rx), List) :- append([H|List1], List2, List),
                            re_match(Rx, [H|List1]),
                            re_match(star(Rx), List2).

to force re_match to "eat something" when it iterates on the star construct.

Answer 2

Consider using DCG notation for better readability and to more easily reason about termination properties:

:- op(100, xf, *).

rexp(eps)      --> [].
rexp([T])      --> [T].
rexp(_*)       --> [].
rexp(R*)       --> rexp(R), rexp(R*).
rexp(s(R1,R2)) --> rexp(R1), rexp(R2).
rexp((R1|R2))    --> ( rexp(R1) ; rexp(R2) ).

Example using length/2 to generate increasingly longer lists to generate strings that are matched by the regexp:

?- length(Ls, _), phrase(rexp(s(([a]|[b]),[c]*)), Ls).
Ls = [a] ;
Ls = [b] ;
Ls = [a, c] ;
Ls = [b, c] ;
Ls = [a, c, c] ;
etc.