How to know what re.sub took?

[Darrell]

"Per Kistler"
> How to know what has been substituted away, if one does something like:
>
>  >>> import re
>  >>> rex = re.compile(r"(hans|fred|simon)")
>  >>> st = "max hans bill"
>  >>> st2 = rex.sub("",st)
>  >>> st2
> 'max bill'
>
> It took "hans" away, but how can I learn this automatically?
>
This won't work for stuff like \g<1> in the replacement string.

import re
def sub(pat, repl, string):
    areas=[]
    def detect(mo, repl=repl, areas=areas):
        areas.append(mo.groups())
        return repl

    return re.sub(pat, detect, string), areas

print sub(r"(hans|fred|simon)","","max hans bill")


('max  bill', [('hans',)])