\begin{frame}{Lexical Analysis}
  \begin{goal}{}
    How to get from characters to tokens?
  \end{goal}
      
  \begin{itemize}
  \pause
    \item 
      Regular expressions $r_1,\ldots,r_n$ express the pattern.
      \smallskip
      
      \emph{Every regular expression corresponds to a token.}
  \pause
    \item 
      Lexical analysis repeatedly searches the \emph{longest prefix} of the input
      that is matched by one of the regular expressions.
      This prefix is transformed into a token.
%       \pause\smallskip
%       
%       When no prefix matches, the result is an error message.
%       \pause\smallskip
%       
%       When there are multiple longest prefixes, one is chosen.
  \end{itemize}
  \pause
  
  \begin{goal}{}
    For \emph{improved performance}:
    \begin{itemize}
      \item 
        Regular expressions are translated \emph{minimal DFAs}.
    \end{itemize}
  \end{goal}
  \pause  
  \begin{exampleblock}{}
    Parser/lexer generators like 
    \begin{itemize}
      \item JavaCC
      \item LEX
    \end{itemize}
    generate the lexer automatically.
    Thereby regular expressions or grammars 
    are converted to minimal DFAs.
  \end{exampleblock}  
\end{frame}