add simple example of avoiding backtracking
diff --git a/Doc/lib/libre.tex b/Doc/lib/libre.tex
index 1c14072..ab51c10 100644
--- a/Doc/lib/libre.tex
+++ b/Doc/lib/libre.tex
@@ -802,10 +802,6 @@
\subsection{Examples}
-%\begin{list}{}{\leftmargin 0.7in \labelwidth 0.65in}
-
-%\item[Simulating scanf]
-
\leftline{\strong{Simulating \cfunction{scanf()}}}
Python does not currently have an equivalent to \cfunction{scanf()}.
@@ -851,7 +847,27 @@
The equivalent regular expression would be
\begin{verbatim}
- ([^\s]+) - (\d+) errors, (\d+) warnings
+ (\S+) - (\d+) errors, (\d+) warnings
\end{verbatim}
-%\end{list}
+\leftline{\strong{Avoiding backtracking}}
+
+If you create regular expressions that require the engine to perform a lot
+of backtracking, you may encounter a RuntimeError exception with the message
+\code{maximum recursion limit exceeded}. For example,
+
+\begin{verbatim}
+ >>> s = "<" + "that's a very big string!"*1000 + ">"
+ >>> re.match('<.*?>', s)
+ Traceback (most recent call last):
+ File "<stdin>", line 1, in ?
+ File "/usr/local/lib/python2.3/sre.py", line 132, in match
+ return _compile(pattern, flags).match(string)
+ RuntimeError: maximum recursion limit exceeded
+\end{verbatim}
+
+You can often restructure your regular expression to avoid backtracking.
+The above regular expression can be recast as \regexp{\textless
+[\textasciicircum \textgreater]*\textgreater}. As a further
+benefit, such regular expressions will run faster than their backtracking
+equivalents.