How To Sanitize Domain ?

[saversites]

Hi,

I have a form field where I prompt user for tgheir domain names. Now, I need to filter it so user inputs valid domain. How to do it ? This ain't working:

$primary_website_domain = filter_var(trim($_POST["primary_website_domain"],FILTER_SANITIZE_DOMAIN)); 
        $primary_website_domain_confirmation = filter_var(trim($_POST["primary_website_domain_confirmation"],FILTER_SANITIZE_DOMAIN));