[Python] Regular expression using group reference

I have a regular expression but do not know what does it means:

>>> import re
>>> re.sub(u'(^| )'0,u'\g<1>null','0fa0a')


I expect it to replace the begin of string or empty space will null, but actual does nothing.

>>> '0fa0a'


What does the group reference refer to ? I haven't defined group 1. Why does it not replace?

Continue reading...