判断IE浏览器用IE条件表达式

<!--[if IE]>
   <script type="text/javascript">
      alert("ie")
   </script>
< ![endif]--> 

<!--[if IE 6]>
    <script type="text/javascript">
       alert("ie6")
    </script>
< ![endif]--> 

<!--[if IE 7]>
     <script type="text/javascript">
         alert("ie7")
     </script>
< ![endif]-->