Domain whitelist for buddypress signup

I am trying to add a domain whitelist to the buddypress registration.

all of the email address that will sign up to on my site will have the domain form @XX.yo.ac.uk with the XX varying in character length and number characters themselves.

I basically would like to strip the domain of the entered email address down to the last 9 characters and then compare them to (.yo.ac.uk) and return an error message if not identical, if not then allow the registration to continue.

I found this code:

// ====================================
// Limit signup by banning domains
function bp_as_restrict_signup_domains( $result ) {

 $banned = array(
 'spam1.com',
 'spam2.com'
 );
 $error = 'Your email domain has been the source of spam. Please use another email address.';

 $email = $result['user_email'];
 $domain = array_pop(explode('@', $email));
if ( in_array($domain, $banned))
 {
 $result['errors']->add('user_email', __($error, 'bp-restrict-email-domains' ) );
 };
 return $result;
}
add_filter( 'bp_core_validate_user_signup', 'bp_as_restrict_signup_domains' );

here: https://alexschreyer.net/programming/limit-buddypress-registration-by-domain/

But this is to block certain domains rather than allow. Any ideas how to change it? and to strip out the characters before the last 9?