function sanitize_trackback_urls( $to_ping ) {
	$urls_to_ping = preg_split( '/[\r\n\t ]/', trim( $to_ping ), -1, PREG_SPLIT_NO_EMPTY );
	foreach ( $urls_to_ping as $k => $url ) {
		if ( ! preg_match( '#^https?://.#i', $url ) ) {
			unset( $urls_to_ping[ $k ] );
		}
	}
	$urls_to_ping = array_map( 'esc_url_raw', $urls_to_ping );
	$urls_to_ping = implode( "\n", $urls_to_ping );
	/**
	 * Filters a list of trackback URLs following sanitization.
	 *
	 * The string returned here consists of a space or carriage return-delimited list
	 * of trackback URLs.
	 *
	 * @since 3.4.0
	 *
	 * @param string $urls_to_ping Sanitized space or carriage return separated URLs.
	 * @param string $to_ping      Space or carriage return separated URLs before sanitization.
	 */
	return apply_filters( 'sanitize_trackback_urls', $urls_to_ping, $to_ping );
}