Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 18235)
+++ wp-includes/functions.php	(working copy)
@@ -2129,7 +2129,7 @@
 		return true;
 
 	// a path starting with / or \ is absolute; anything else is relative
-	return (bool) preg_match('#^[/\\\\]#', $path);
+	return ( $path[0] == '/' || $path[0] == '\\' );
 }
 
 /**