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<title>4.2.6 Examples
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<H2><A NAME=
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<DIV CLASS=
"leftline" ID=
"par86950" ALIGN=
"LEFT">
<strong>Simulating
<tt class=
"cfunction">scanf()
</tt></strong></DIV>
Python does not currently have an equivalent to
<tt class=
"cfunction">scanf()
</tt>.
<a id='l2h-
914' xml:id='l2h-
914'
></a>
Regular expressions are generally more powerful, though also more
verbose, than
<tt class=
"cfunction">scanf()
</tt> format strings. The table below
offers some more-or-less equivalent mappings between
<tt class=
"cfunction">scanf()
</tt> format tokens and regular expressions.
<div class=
"center"><table class=
"realtable">
<th class=
"left" ><tt class=
"cfunction">scanf()
</tt> Token
</th>
<th class=
"left" >Regular Expression
</th>
<tr><td class=
"left" valign=
"baseline"><code>%c
</code></td>
<td class=
"left" ><tt class=
"regexp">.
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%
5c
</code></td>
<td class=
"left" ><tt class=
"regexp">.{
5}
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%d
</code></td>
<td class=
"left" ><tt class=
"regexp">[-+]?
\d+
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%e
</code>,
<code>%E
</code>,
<code>%f
</code>,
<code>%g
</code></td>
<td class=
"left" ><tt class=
"regexp">[-+]?(
\d+(
\.
\d*)?|
\d*
\.
\d+)([eE][-+]?
\d+)?
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%i
</code></td>
<td class=
"left" ><tt class=
"regexp">[-+]?(
0[xX][
\dA-Fa-f]+|
0[
0-
7]*|
\d+)
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%o
</code></td>
<td class=
"left" ><tt class=
"regexp">0[
0-
7]*
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%s
</code></td>
<td class=
"left" ><tt class=
"regexp">\S+
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%u
</code></td>
<td class=
"left" ><tt class=
"regexp">\d+
</tt></td></tr>
<tr><td class=
"left" valign=
"baseline"><code>%x
</code>,
<code>%X
</code></td>
<td class=
"left" ><tt class=
"regexp">0[xX][
\dA-Fa-f]+
</tt></td></tr></tbody>
To extract the filename and numbers from a string like
<div class=
"verbatim"><pre>
/usr/sbin/sendmail -
0 errors,
4 warnings
you would use a
<tt class=
"cfunction">scanf()
</tt> format like
<div class=
"verbatim"><pre>
%s - %d errors, %d warnings
The equivalent regular expression would be
<div class=
"verbatim"><pre>
(\S+) - (\d+) errors, (\d+) warnings
<DIV CLASS=
"leftline" ID=
"par86951" ALIGN=
"LEFT">
<strong>Avoiding recursion
</strong></DIV>
If you create regular expressions that require the engine to perform a
lot of recursion, you may encounter a RuntimeError exception with
the message
<code>maximum recursion limit
</code> exceeded. For example,
<div class=
"verbatim"><pre>
>>> s = 'Begin ' +
1000*'a very long string ' + 'end'
>>> re.match('Begin (\w| )*? end', s).end()
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
You can often restructure your regular expression to avoid recursion.
Starting with Python
2.3, simple uses of the
<tt class=
"regexp">*?
</tt> pattern are
special-cased to avoid recursion. Thus, the above regular expression
can avoid recursion by being recast as
<tt class=
"regexp">Begin [a-zA-Z0-
9_ ]*?end
</tt>. As a further benefit, such regular
expressions will run faster than their recursive equivalents.
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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