#!/usr/bin/perl -w
use strict;
while( my $string=<DATA>)
  { my( $domain)= ($string=~ m{(?:^|\.)       # the beginning of the string or .
                               ([^.]*)        # anything but . (and store it in $1)
                               \.com          # .com
                              (?:\/|$)       # a / or the end of the string
                              }x);
    print "domain: $domain\n";
  }

__DATA__
l-12345.in.some.domain.com
l-12345.in.some.domain.com/blargh/index.html
domain.com/blargh/index.html
domain.com
domain.com/
l-12345.in.some.domain.com/blargh/foo.com
nope.com.domain.com/blargh/foo.com
l-12345.in.some.domain.com/blargh/nope.foo.com