Regex comparison #32

We could have complete support for comparing regexes (in e.g. the pattern field in json schema), if we compiled the regexes to automata and did set-theoretic operations on the automata (product with intersection, product with union, negation), and used an emptiness test for an automaton. These operations can be carried out somewhat efficiently. After talking a little bit with ekmett about this he suggested that we use Binary decision diagrams and in particular ZDD, for determinization (NFA -> DFA) and NFA membership test.