\begin{frame}{Word (String) Matching (Thompson, 1968)}
  \begin{alertblock}{}
    The standard regular expression libraries of 
    \begin{itemize}
      \item Java, 
      \item Perl, 
      \item PHP, 
      \item Python
    \end{itemize}
    \emph{do not use the efficient algorithm} from the last slide.
    \bigskip
    
    They use a \emph{backtracking} algorithm with worst-case complexity
    \begin{center}
      \alert{exponential time} (in $|u|$)
    \end{center}
  \end{alertblock}
\end{frame}