--- a/slides/slides04.tex	Mon Oct 08 11:35:04 2018 +0100
+++ b/slides/slides04.tex	Tue Oct 09 08:16:25 2018 +0100
@@ -32,7 +32,26 @@
   \end{tabular}
   \end{center}
 \end{frame}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+\frametitle{Regular Expressions}
+
+In programming languages they are often used to recognise:\medskip
+
+\begin{itemize}
+\item symbols, digits
+\item identifiers
+\item numbers (non-leading zeros)
+\item keywords
+\item comments
+\end{itemize}\bigskip
+
+\mbox{}\hfill\bl{\url{http://www.regexper.com}}
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   
+
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \begin{frame}[c]