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Via Ned Batchelder, an article on Reversing Regular Expressions from Perl.com. Otherwise known as Sexeger, these offer a performance boost over normal regular expressions for certain tasks. The basic idea is pretty simple: searching backwards through a string using a regular expression can be a messy business, but by reversing both the string and the expression, running it, then reversing the result far better performance can be achieved (reversing a string is a relatively inexpensive operation). The example code is in Perl, but I couldn't resist trying it in Python. The challenge is to find the last number occurring in a string.

>>> import re
>>> lastnum = re.compile(r'(\d+)(?!\D*\d)')
>>> s = ' this isa 454 asd very very 
  very long strin9 asd9 009 76 with numbers 
  99 in it and here is the last 537 number'
  # NB this was all on one line originally
>>> lastnum.search(s).group(0)
'537'
>>> import timeit
>>> t1 = timeit.Timer('lastnum.search(s).group(0)', 
         'from __main__ import lastnum, s')
>>> print "%.2f usec/pass" % (1000000 * t1.timeit(number=100000)/100000)
26.82 usec/pass
>>> lastnumrev = re.compile('(\d+)')
>>> lastnumrev.search(s[::-1]).group(0)[::-1]
'537'
>>> t2 = timeit.Timer('lastnumrev.search(s[::-1]).group(0)[::-1]', 
         'from __main__ import lastnumrev, s')
>>> print "%.2f usec/pass" % (1000000 * t2.timeit(number=100000)/100000)
9.26 usec/pass

There are a few points worth explaining in the above code. The (?!\D*\d) part of the first regular expression is a negative lookahead assertion - it basically means "match the subpattern provided it isn't followed by a string of non-digits followed by at least one digit. This is the bit that ensures we only get back the last digit in the string, and is also the bit that could cause a performance problem.

'some string'[::-1] is an example of Extended Slices, introduced in Python 2.3. Its affect is to reverse the string, by stepping through it from start to end going back one character at a time.

The actual benchmarking code makes use of the new timeit module from Python 2.3 - I copied it verbatim from that module's example section in the manual.

The resutls speak for themselves: 26.82 for the lookahead assertion expression compared to just 9.26 for the reversed regular expression. This is definitely a useful trick to add to the tool box.