1. Compile a hash/array of common TLDs
   (there are several good lists out there on the web)
2. Strip off TLD from domain (ie, .com, .co.uk, .la)
   sub.domain.tld => sub.domain
3. Last dot-token must be domain
   sub.domain => domain
4. Re-append TLD
   domain => domain.tld

I understand processing speed is a factor, but with a little optimization... =)

Good luck.

'kaboo

Update:
Here's a start. (I was bored.)

sub parse_fqdn {
    my $fqdn = shift;
    my @tlds = (
        [qw(com net org)],    # common
        [qw(gov edu co\.uk)], # not-so-common
        [qw(la li it po)]     # downright odd (long list)
    );

    foreach my $level (@tlds) {
        #warn "pass ". ++$pass;
        for(@{$level}) {
            return $1 if $fqdn =~ /\.(\w+?\.$_)$/;
        }
    }

    warn "unable to find domain from $fqdn\n";
    return $fqdn;
}

my $domain = parse_fqdn('www.cs.niu.edu'); 
print "www.cs.niu.edu => $domain\n"; # niu.edu
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