\begin{frame}{Regular Expressions $\iff$ Regular Languages}
  \begin{block}{Theorem}
  A language $L$ is \emph{regular} \\
  \hfill $\iff$ there is a \emph{regular expression} $r$ with $L(r) = L$.
  \end{block}
  \pause\medskip
  
  \begin{proof}
    We need to prove two directions:
    \begin{itemize}
      \medskip
      \item $(\Leftarrow)$
        Translate regular expressions into NFAs.
      \medskip
      \item $(\Rightarrow)$
        Translate NFAs into regular expressions.
      \medskip
    \end{itemize}
  \end{proof}
\end{frame}