Conversion of regular expressionsinto realtime automata

Abstract

We consider conversions of regular expressions into k-realtimefinite state automata, i.e., automata in which the number ofconsecutive uses of ε-transitions, along any computation path,is bounded by a fixed constant k. For 2-realtime automata,i.e., for automata that cannot change the state, without readingan input symbol, more than two times in a row, we show that theconversion of a regular expression into such an automaton producesonly O(n) states, O(nlogn) ε-transitions, and O(n)alphabet-transitions. We also show how to easily transform these2-realtime machines into 1-realtime automata, still with onlyO(nlogn) edges. These results contrast with the known lowerbound Ω(n(logn)² / loglogn), holding for 0-realtimeautomata, i.e., for automata with no ε-transitions.