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| -rw-r--r-- | Doc/lib/libre.tex | 28 |
1 files changed, 22 insertions, 6 deletions
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 @@ The string passed to \function{match()} or \function{search()}. \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 @@ you would use a \cfunction{scanf()} format like The equivalent regular expression would be \begin{verbatim} - ([^\s]+) - (\d+) errors, (\d+) warnings + (\S+) - (\d+) errors, (\d+) warnings +\end{verbatim} + +\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} -%\end{list} +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. |