The fact that regular languages are closed under Boolean operations simpli?es the process of establishing regularity of languages; in essence we can augment the regular operations with intersection and complement (as well as any other operations we can show preserve regularity). All one need do to prove a language is regular, then, is to show how to construct it from "obviously" regular languages using any of these operations. (A little care is needed about what constitutes "obvious". The safest thing to do is to take the language back all the way to ∅, {ε}, and the singleton languages of unit strings.)