regexp in Python (from Perl)

[Pat]

I have a regexp in Perl that converts the last digit of an ip address to 
  '9'.  This is a very particular case so I don't want to go off on a 
tangent of IP octets.

  ( my $s = $str ) =~ s/((\d+\.){3})\d+/${1}9/ ;

While I can do this in Python which accomplishes the same thing:

ip = ip[ :-1 ]
ip =+ '9'

I'm more interested, for my own edification in non-trivial cases, in how 
one would convert the Perl RE to a Python RE that use groups.  I am 
somewhat familiar using the group method from the re package but I 
wanted to know if there was a one-line solution.

Thank you.