Source

File: wp-includes/kses.php

function wp_kses_bad_protocol_once( $string, $allowed_protocols, $count = 1 ) {
	$string  = preg_replace( '/(&#0*58(?![;0-9])|&#x0*3a(?![;a-f0-9]))/i', '$1;', $string );
	$string2 = preg_split( '/:|&#0*58;|&#x0*3a;|:/i', $string, 2 );
	if ( isset( $string2[1] ) && ! preg_match( '%/\?%', $string2[0] ) ) {
		$string   = trim( $string2[1] );
		$protocol = wp_kses_bad_protocol_once2( $string2[0], $allowed_protocols );
		if ( 'feed:' === $protocol ) {
			if ( $count > 2 ) {
				return '';
			}
			$string = wp_kses_bad_protocol_once( $string, $allowed_protocols, ++$count );
			if ( empty( $string ) ) {
				return $string;
			}
		}
		$string = $protocol . $string;
	}

	return $string;
}