Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 21543)
+++ wp-includes/formatting.php	(working copy)
@@ -3235,7 +3235,7 @@
  * @return string URLs starting with the http or https protocol, separated by a carriage return.
  */
 function sanitize_trackback_urls( $to_ping ) {
-	$urls_to_ping = preg_split( '/\r\n\t /', trim( $to_ping ), -1, PREG_SPLIT_NO_EMPTY );
+	$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] );