Regular realizability problems and regular languages

Abstract: We investigate regular realizability (RR) problems, which are the problems of verifying whether intersection of a regular language -- the input of the problem -- and fixed language called filter is non-empty. We consider two kind of problems depending on representation of regular language. If a regular language on input is represented by a DFA, then we obtain (deterministic) regular realizability problem and we show that in this case the complexity of regular realizability problem for an arbitrary regular filter is either L-complete or NL-complete. We also show that in case of representation regular language on input by NFA the problem is always NL-complete.