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4.2.6 Examples

Python does not currently have an equivalent to scanf().  Regular expressions are generally more powerful, though also more verbose, than scanf() format strings. The table below offers some more-or-less equivalent mappings between scanf() format tokens and regular expressions.

scanf() Token	Regular Expression
%c	.
%5c	.{5}
%d	[-+]?\d+
%e, %E, %f, %g	[-+]?(\d+(\.\d*)?|\d*\.\d+)([eE]\d+)?
%i	[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)
%o	0[0-7]*
%s	\S+
%u	\d+
%x, %X	0[xX][\dA-Fa-f]+

To extract the filename and numbers from a string like

    /usr/sbin/sendmail - 0 errors, 4 warnings

you would use a scanf() format like

    %s - %d errors, %d warnings

The equivalent regular expression would be

    (\S+) - (\d+) errors, (\d+) warnings

If you create regular expressions that require the engine to perform a lot of recursion, you may encounter a RuntimeError exception with the message maximum recursion limit exceeded. For example,

>>> import re
>>> 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 *? pattern are special-cased to avoid recursion. Thus, the above regular expression can avoid recursion by being recast as Begin [a-zA-Z0-9_ ]*?end. As a further benefit, such regular expressions will run faster than their recursive equivalents.

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Python Library Reference

Previous: 4.2.5 Match Objects Up: 4.2 re Next: 4.3 struct

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Release 2.3, documentation updated on July 29, 2003.