Stop users from setting their username to an email address with Paid Memberships Pro

pmpro_registration_checks_no_email_user_login.php

/*
	Stop users from setting their username to an email address
	Add this code to a custom plugin.
*/
function pmpro_registration_checks_no_email_user_login($continue) {
	//if there are earlier problems, don't bother checking
	if(!$continue)
		return;

	//make sure the username passed in doesn't look like an email address (contains a @)
	global $username;
	if(!empty($username) && strpos($username, "@") !== false) {
		$continue = false;
		pmpro_setMessage( 'Your username must not be an email address', 'pmpro_error' );
	}

	return $continue;
}
add_filter('pmpro_registration_checks', 'pmpro_registration_checks_no_email_user_login');

This recipe is included in the blog post on "Prevent users from using an email address as their username." at Paid Memberships Pro here: https://www.paidmembershipspro.com/prevent-users-from-using-an-email-address-as-username/