Help with regexp please

[Felix Collins]

Hi,
  I'm not a regexp expert and had a bit of trouble with the following 
search.

I have an "outline number" system like

1
1.2
1.2.3
1.3
2
3
3.1

etc.

I want to parse an outline number and return the parent.

So for example...

parent("1.2.3.4") returns "1.2.3"

The only way I can figure is to do two searches feeding the output of 
the first into the input of the second.

Here is the code fragment...

m = re.compile(r'(\d+\.)+').match("1.2.3.4")
n = re.compile(r'\d+(\.\d+)+').match(m.string[m.start():m.end()])
parentoutlinenumber = n.string[n.start():n.end()]

parentoutlinenumber
1.2.3

How do I get that into one regexp?

Thanks for any help...

Felix