reusing parts of a string in RE matches?

[Ben Cartwright]

John Salerno wrote:
> So my question is, how can find all occurrences of a pattern in a
> string, including overlapping matches? I figure it has something to do
> with look-ahead and look-behind, but I've only gotten this far:
>
> import re
> string = 'abababababababab'
> pattern = re.compile(r'ab(?=a)')
> m = pattern.findall(string)
>
> This matches all the 'ab' followed by an 'a', but it doesn't include the
> 'a'. What I'd like to do is find all the 'aba' matches. A regular
> findall() gives four results, but really there are seven.
>
> Is there a way to do this with just an RE pattern, or would I have to
> manually add the 'a' to the end of the matches?

Yes, and no extra for loops are needed!  You can define groups inside
the lookahead assertion:

  >>> import re
  >>> re.findall(r'(?=(aba))', 'abababababababab')
  ['aba', 'aba', 'aba', 'aba', 'aba', 'aba', 'aba']

--Ben