4.2.6 Examples
Simulating scanf()
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.
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
Avoiding recursion
If you create regular expressions that require the engine to perform a
lot of recursion, you may encounter a RuntimeError exception with
the message
>>> 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/python2.5/re.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. See About this document... for information on suggesting changes. |