Regular relations, which are modeled by finite-state transducers and are very closely related to finite-state machines of the standard kind, have just the power that is needed for many linguistic operations, particularly at the low end of the hierarchyphonology, morphology, and spelling rules.
as Ron and I pointed out (Kay and Kaplan 1994), a slight adjustment to the rules of engagement, as they are called in military circles these days, moves simple string-rewriting rules right from the top of the Chomsky hierarchy to the bottom or, at least, one step from the bottom.