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<H2><A NAME="SECTION006260000000000000000">
4.2.6 Examples</A>
</H2>

<P>
<DIV CLASS="leftline" ID="par86950" ALIGN="LEFT">
<strong>Simulating <tt class="cfunction">scanf()</tt></strong></DIV>

<P>
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.

<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="left"  ><tt class="cfunction">scanf()</tt> Token</th>
      <th class="left"  >Regular Expression</th>
      </tr>
    </thead>
  <tbody>
    <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">[-+]?&#92;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">[-+]?(&#92;d+(&#92;.&#92;d*)?|&#92;d*&#92;.&#92;d+)([eE][-+]?&#92;d+)?</tt></td></tr>
    <tr><td class="left"   valign="baseline"><code>%i</code></td>
        <td class="left"  ><tt class="regexp">[-+]?(0[xX][&#92;dA-Fa-f]+|0[0-7]*|&#92;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">&#92;S+</tt></td></tr>
    <tr><td class="left"   valign="baseline"><code>%u</code></td>
        <td class="left"  ><tt class="regexp">&#92;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][&#92;dA-Fa-f]+</tt></td></tr></tbody>
</table></div>

<P>
To extract the filename and numbers from a string like

<P>
<div class="verbatim"><pre>
    /usr/sbin/sendmail - 0 errors, 4 warnings
</pre></div>

<P>
you would use a <tt class="cfunction">scanf()</tt> format like

<P>
<div class="verbatim"><pre>
    %s - %d errors, %d warnings
</pre></div>

<P>
The equivalent regular expression would be

<P>
<div class="verbatim"><pre>
    (\S+) - (\d+) errors, (\d+) warnings
</pre></div>

<P>
<DIV CLASS="leftline" ID="par86951" ALIGN="LEFT">
<strong>Avoiding recursion</strong></DIV>

<P>
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,

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import re
&gt;&gt;&gt; s = 'Begin ' + 1000*'a very long string ' + 'end'
&gt;&gt;&gt; re.match('Begin (\w| )*? end', s).end()
Traceback (most recent call last):
  File "&lt;stdin&gt;", 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
</pre></div>

<P>
You can often restructure your regular expression to avoid recursion.

<P>
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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	53
	54	<H2><A NAME="SECTION006260000000000000000">
	55	4.2.6 Examples</A>
	56	</H2>
	57
	58	<P>
	59	<DIV CLASS="leftline" ID="par86950" ALIGN="LEFT">
	60	<strong>Simulating <tt class="cfunction">scanf()</tt></strong></DIV>
	61
	62	<P>
	63	Python does not currently have an equivalent to <tt class="cfunction">scanf()</tt>.
	64	<a id='l2h-914' xml:id='l2h-914'></a>
65	Regular expressions are generally more powerful, though also more
66	verbose, than <tt class="cfunction">scanf()</tt> format strings. The table below
67	offers some more-or-less equivalent mappings between
68	<tt class="cfunction">scanf()</tt> format tokens and regular expressions.
69
70	<P>
71	<div class="center"><table class="realtable">
72	<thead>
73	<tr>
74	<th class="left" ><tt class="cfunction">scanf()</tt> Token</th>
75	<th class="left" >Regular Expression</th>
76	</tr>
77	</thead>
78	<tbody>
79	<tr><td class="left" valign="baseline"><code>%c</code></td>
80	<td class="left" ><tt class="regexp">.</tt></td></tr>
81	<tr><td class="left" valign="baseline"><code>%5c</code></td>
82	<td class="left" ><tt class="regexp">.{5}</tt></td></tr>
83	<tr><td class="left" valign="baseline"><code>%d</code></td>
84	<td class="left" ><tt class="regexp">[-+]?\d+</tt></td></tr>
85	<tr><td class="left" valign="baseline"><code>%e</code>, <code>%E</code>, <code>%f</code>, <code>%g</code></td>
86	<td class="left" ><tt class="regexp">[-+]?(\d+(\.\d)?\|\d\.\d+)([eE][-+]?\d+)?</tt></td></tr>
87	<tr><td class="left" valign="baseline"><code>%i</code></td>
88	<td class="left" ><tt class="regexp">[-+]?(0[xX][\dA-Fa-f]+\|0[0-7]*\|\d+)</tt></td></tr>
89	<tr><td class="left" valign="baseline"><code>%o</code></td>
90	<td class="left" ><tt class="regexp">0[0-7]*</tt></td></tr>
91	<tr><td class="left" valign="baseline"><code>%s</code></td>
92	<td class="left" ><tt class="regexp">\S+</tt></td></tr>
93	<tr><td class="left" valign="baseline"><code>%u</code></td>
94	<td class="left" ><tt class="regexp">\d+</tt></td></tr>
95	<tr><td class="left" valign="baseline"><code>%x</code>, <code>%X</code></td>
96	<td class="left" ><tt class="regexp">0[xX][\dA-Fa-f]+</tt></td></tr></tbody>
97	</table></div>
98
99	<P>
100	To extract the filename and numbers from a string like
101
102	<P>
103	<div class="verbatim"><pre>
104	/usr/sbin/sendmail - 0 errors, 4 warnings
105	</pre></div>
106
107	<P>
108	you would use a <tt class="cfunction">scanf()</tt> format like
109
110	<P>
111	<div class="verbatim"><pre>
112	%s - %d errors, %d warnings
113	</pre></div>
114
115	<P>
116	The equivalent regular expression would be
117
118	<P>
119	<div class="verbatim"><pre>
120	(\S+) - (\d+) errors, (\d+) warnings
121	</pre></div>
122
123	<P>
124	<DIV CLASS="leftline" ID="par86951" ALIGN="LEFT">
125	<strong>Avoiding recursion</strong></DIV>
126
127	<P>
128	If you create regular expressions that require the engine to perform a
129	lot of recursion, you may encounter a RuntimeError exception with
130	the message <code>maximum recursion limit</code> exceeded. For example,
131
132	<P>
133	<div class="verbatim"><pre>
134	>>> import re
135	>>> s = 'Begin ' + 1000*'a very long string ' + 'end'
136	>>> re.match('Begin (\w\| )*? end', s).end()
137	Traceback (most recent call last):
138	File "<stdin>", line 1, in ?
139	File "/usr/local/lib/python2.3/sre.py", line 132, in match
140	return _compile(pattern, flags).match(string)
141	RuntimeError: maximum recursion limit exceeded
142	</pre></div>
143
144	<P>
145	You can often restructure your regular expression to avoid recursion.
146
147	<P>
148	Starting with Python 2.3, simple uses of the <tt class="regexp">*?</tt> pattern are
149	special-cased to avoid recursion. Thus, the above regular expression
150	can avoid recursion by being recast as
151	<tt class="regexp">Begin [a-zA-Z0-9_ ]*?end</tt>. As a further benefit, such regular
152	expressions will run faster than their recursive equivalents.
153
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