To copmly with RFC 1123, I used what was posted here: http://stackoverflow.com/questions/106179/regular-expression-to-match-hostname-or-ip-address

Here's my subroutine for perl (seems to work nicely):

sub is_FQDN {
   my $testval = shift(@_);                                
   ( $testval =~ m/^(([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]*[a-zA-Z0-9]
+)\.)*([A-Za-z0-9]|[A-Za-z0-9][A-Za-z0-9\-]*[A-Za-z0-9])$/ )
   ? return 1
   : return 0;
}
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