Hello all! I have the following form used to search for domain names in my website. I don't want the form to submit if the client enters a tld eg: .com .org or .net.

Code:
    <form action="http://www.mydomain.com/clients/whmcs/cart.php?a=add&domain=register" method="post">
    <input type="hidden" name="token" value="4445754545" />
    <input type="hidden"
...