regex error - nothing to repeat

Question

I get an error message when I use this expression:

re.sub(r"([^\s\w])(\s*\1)+","\\1","...") 

I checked the regex at RegExr and it returns . as expected. But when I try it in Python I get this error message:

raise error, v # invalid expression sre_constants.error: nothing to repeat 

Can someone please explain?

Answer 1

It seems to be a python bug (that works perfectly in vim). The source of the problem is the (\s*...)+ bit. Basically , you can't do (\s*)+ which make sense , because you are trying to repeat something which can be null.

>>> re.compile(r"(\s*)+") Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py", line 180, in compile     return _compile(pattern, flags)   File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/re.py", line 233, in _compile     raise error, v # invalid expression sre_constants.error: nothing to repeat 

However (\s*\1) should not be null, but we know it only because we know what's in \1. Apparently python doesn't ... that's weird.