Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 43327)
+++ wp-includes/formatting.php	(working copy)
@@ -3163,7 +3163,7 @@
 			$content = $textarr[ $i ];
 
 			// If we're in an ignore block, wait until we find its closing tag
-			if ( '' == $ignore_block_element && preg_match( '/^<(' . $tags_to_ignore . ')>/', $content, $matches ) ) {
+			if ( '' == $ignore_block_element && preg_match( '/^<(' . $tags_to_ignore . ').*>/U', $content, $matches ) ) {
 				$ignore_block_element = $matches[1];
 			}