\begin{thebibliography}{HMU01}

\bibitem[HMU01]{Hop1}
J.~E. Hopcroft, R.~Motwani, and J.~D. Ullman.
\newblock {\em {``Introduction to Automata Theory, Language, and
  Computation''}}.
\newblock Addison--Wesley, 2nd edition edition, 2001.

\bibitem[Lin06]{linz1}
Peter Linz.
\newblock {\em An Introductionto Formal Languages and Automata}.
\newblock Jones and Bartlett, $4^{th}$ edition, 2006.

\bibitem[Sip06]{Sipser1}
Michael Sipser.
\newblock {\em Introduction to the Theory of Computation}.
\newblock Thompson Course Technology, second edition, 2006.

\end{thebibliography}