For my graduate level course in Formal Language Theory class at Oklahoma State University we are using Hopcroft, Motwani, and Ulmann's
Introduction to Automata Theory, Languages, and Computation. Why?? First of all, we shouldn't be using an introductory, but let's us forego that battle for an instant.
Even in its second addition, this book is rife with ambiguous definitions, questionable examples, and homework problems which are not exact enough to realistically be assigned to students. All of the content in the first few chapters seems to titter between the ambiguous and the arguably wrong.
Sipser has a wonderful, compact, and exact text which more than adequately covers the material for an introductory work. So, why aren't we using it? Dunno.
