Feature request: Closure operations

[ksf]

Regular languages are closed under complement, intersection, union and difference (because they're sets) as well as under reversal, it would be great to have those available as they provide much expressive and combinatorial power, intersection probably being the one with most practical use.

Complement is trivial on a DFA, just switch all final states to non-final and vice versa.

Union already exists in the guise of alternation, intersection follows by De Morgan's Law, difference is (intersect a (not b), reversal is reversing all edges of an DFA and switching start and end states (you end up with an NFA, then)

...that's not to say that I think it's trivial to do, though. Doing them on graph-based automata would be two hours worth of work, implementing them for online automata is a different kind of beast.

[ulidtko]

Perhaps the reference and ideas here could be useful for this:
https://hackage.haskell.org/package/regexpr-symbolic-0.5/src/RegExpr/RegExprOperations.lhs