Do not allow users to use an email as their username

my_pmpro_registration_checks_no_email_in_username.php

<?php   // do not copy this line.
/**
 * This recipe does will check to to make sure an email address is not used for the username.
 * Please be aware this is a "use-at-own-risk" option.
 * If assistance with the implementation or further customizations is required you
 * may reach out to a local WordPress developer
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

/*
 * Do not allow users to use an email as their username
 */
function my_pmpro_registration_checks_no_email_in_username($okay)
{
	if(!$okay)
		return $okay;		//there are other errors to handle
	
	global $pmpro_msg, $pmpro_msgt;	
	
	$username = $_REQUEST['username'];
	
	//make sure they entered any birthday
	if( ! empty($username) && strpos( $username, '@') !== false )
	{
		$pmpro_msg = "You cannot use an email address as your username.";
		$pmpro_msgt = "pmpro_error";
		return false;
	}

	return $okay;
}
add_filter("pmpro_registration_checks", "my_pmpro_registration_checks_no_email_in_username");