noptin_generate_user_name( string $prefix = '' )

function noptin_generate_user_name( $prefix = '' ) {

	// If prefix is an email, retrieve the part before the email.
	$prefix = strtolower( trim( strtok( $prefix, '@' ) ) );

	// Remove whitespace.
	$prefix = preg_replace( '|\s+|', '_', $prefix );

	// Trim to 8 characters max. 
	$prefix = sanitize_user( $prefix );

	// phpcs:ignore Generic.Commenting.DocComment.MissingShort
	/** @ignore */
	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
	if ( empty( $prefix ) || in_array( strtolower( $prefix ), array_map( 'strtolower', $illegal_logins ), true ) ) {
		$prefix = 'noptin';
	}

	$username = $prefix;
	if ( username_exists( $username ) ) {
		$prefix = $prefix . zeroise( wp_rand( 0, 9999 ), 4 );
		return noptin_generate_user_name( $prefix );
	}

	/**
	 * Filters an autogenerated user_name.
	 *
	 * @since 1.2.3
	 * @param string $prefix      A prefix for the user name. Can be any string including an email address.
	 */
	return apply_filters( 'noptin_generate_user_name', $prefix );
}