Index: /CKEditor/trunk/_source/plugins/dialog/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/dialog/plugin.js	(revision 6993)
+++ /CKEditor/trunk/_source/plugins/dialog/plugin.js	(revision 6994)
@@ -2930,6 +2930,6 @@
 			integerRegex = /^\d*$/,
 			numberRegex = /^\d*(?:\.\d+)?$/,
-			htmlLengthRegex = /^([+-]?((\d*(\.\d+))|(\d*))(px|\%)?)?$/,
-			cssLengthRegex = /^([+-]?((\d*(\.\d+))|(\d*))(px|em|ex|in|cm|mm|pt|pc|\%)?)?$/i;
+			htmlLengthRegex = /^(((\d*(\.\d+))|(\d*))(px|\%)?)?$/,
+			cssLengthRegex = /^(((\d*(\.\d+))|(\d*))(px|em|ex|in|cm|mm|pt|pc|\%)?)?$/i;
 
 		CKEDITOR.VALIDATE_OR = 1;