It depends, if you have the opportunity to build a DFA once, why not do it? It's only a one-time cost. I think the more serious problem is that what most peopl

It depends, if you have the opportunity to build a DFA once, why not do it? It's only a one-time cost. I think the more serious problem is that what most people consider to be a 'regular expression' does not express a regular language, and hence cannot be expressed as a finite state automaton.