Help needed FileUpload Regular Expression validation

Hi,

       I am using asp.net, C#, FrameWork 3.5 with FileUpload Control. i need  to accept all files except EXE, DLL , OCX  Files.

       I need a Regular Expression to validate this files and Show the message.

      Sample Code is :

       ucFileUpload uc = (ucFileUpload)e.Row.FindControl("ucFileUpload1");

       uc.UploadMessage = "Invalid File. File format should not be : .exe|.ocx|dll";

       uc.UploadExpression = ?

  ...