urlparse

foo.sh

#!/usr/bin/env bash

URL="$1"
extract() {
    local url=$1
python << EOF
import urlparse
u = '$url'
pu = urlparse.urlparse(u)
netloc = pu.netloc
tlds = [a.strip() for a in open('tld.txt').read().splitlines() if a.strip()]
step = 2
matching_tlds = [tld for tld in tlds if netloc.endswith(tld)]
matching_tlds.sort(key=lambda x:len(x))
if matching_tlds:
    step += matching_tlds[-1].count('.') -1
print '.'.join(netloc.split('.')[-step:])
EOF
}

echo "$1:         $(extract $1)"

# vim:set et sts=4 ts=4 tw=80:

tld.txt

other.co.uk
co.uk
bar.co.uk

for i in http://test.foo.co.uk http://test.other.co.uk http://test.bar.co.uk http://test.co.uk http://www.foo.net http://foo;do ./foo.sh $i;done
http://test.foo.co.uk: co.uk
http://test.other.co.uk: other.co.uk
http://test.bar.co.uk: bar.co.uk
http://test.co.uk: co.uk
http://www.foo.net: foo.net
http://foo: foo
kiorky@judith:~/gist-412417