Question

Can someone explain in clear language how the non-zero lookbehind assertion (?<=.) technically works in the example below? That is, can you actually walk through it? The code works fine and produces the correct result.

I read about (?<=\w) at http://www.regular-expressions.info/wordboundaries.html, but the explanation was not clear to someone trying to learn about lookbehind assertions. Thanks.

>>> text = 'sassy'   
>>> for (regexp,subst) in [ (r'(?<=.)s', '5'),(r'^s', '$') ]:  
...   text = re.sub(regexp,subst,text)
...  
>>> text
'$a55y'

Answer 1

Look-behind does just that: it verifies that whatever precedes the current character matches the look-behind expression.

In your case, (?<=.)s will match s, but only if what precedes it matches . (i.e. anything except CRLF).