Source

File: wp-includes/kses.php

function wp_kses_bad_protocol( $string, $allowed_protocols ) {
	$string     = wp_kses_no_null( $string );
	$iterations = 0;

	do {
		$original_string = $string;
		$string          = wp_kses_bad_protocol_once( $string, $allowed_protocols );
	} while ( $original_string != $string && ++$iterations < 6 );

	if ( $original_string != $string ) {
		return '';
	}

	return $string;
}