re.sub(): replace longest match instead of leftmost match?

[John Gordon]

According to the documentation on re.sub(), it replaces the leftmost
matching pattern.

However, I want to replace the *longest* matching pattern, which is
not necessarily the leftmost match.  Any suggestions?

I'm working with IPv6 CIDR strings, and I want to replace the longest
match of "(0000:|0000$)+" with ":".  But when I use re.sub() it replaces
the leftmost match, even if there is a longer match later in the string.

I'm also looking for a regexp that will remove leading zeroes in each
four-digit group, but will leave a single zero if the group was all
zeroes.

Thanks!

-- 
John Gordon                   A is for Amy, who fell down the stairs
gordon at panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb Tinies"