require minimum length for WordPress user login

gistfile1.txt

/**
 * Additional registration checks.
 *
 * This filter is documented in wp-includes/user.php
 */
function my_registration_errors( $errors, $sanitized_user_login, $user_email ) {
	if ( strlen( $sanitized_user_login ) < 5 ) {
		$errors->add( 'username_too_short', __( '<strong>ERROR</strong>: Username must be at least 5 characters.' ) );
	}
	return $errors;
}
add_filter( 'registration_errors', 'my_registration_errors', 10, 3 );