Regular expression problem with groups

[Peter Otten]

Axel Kowald wrote:

> Hi everybody,
> 
> I have a 'simple' problem with regular expressions. Maybe someone can
> help me.
> 
> import re
> bla = 'the the'
> obj = re.search('(.+) \1',bla)
> 
> obj.group(1) should now be 'the'. Instead obj is none (python 2.3
> under windows and 2.1 under linux). The problem seems to be the \1
> where I try to reference the first group. However, every tutorial says
> this is the correct way to do it !?
> 
> Any ideas what I'm doing wrong ?

You did neither escape the backslash nor use a raw string.

>>> re.search("(.+) \\1", "the the").group(1)
'the'

>>> re.search(r"(.+) \1", "the the").group(1)
'the'

The latter is the most convenient way to use with regular expressions with
the limitation that it may not end with an odd number of backslashes.

Peter