1. L(M') is regular: in that case L membership is about equivalence of DFA's (there exists a DFA for L(M')) which was shown in class to be decidable

2. L(M') is not regular: in that case L is empty and hence decidable.

As for the question regarding whether given M' is L(M') regular - this is undecidable (not even in RE U co-RE)

]]>Is L decidable?

Is there a way to determine whether L(M') is REGULAR? ]]>