禁止IE backspace

    $(document).keydown(function(e){  
       var target = e.target ;  
       var tag = e.target.tagName.toUpperCase();  
       if(e.keyCode == 8){  
        if((tag == 'INPUT' && !$(target).attr("readonly"))||(tag == 'TEXTAREA' && !$(target).attr("readonly"))){  
         if((target.type.toUpperCase() == "RADIO") || (target.type.toUpperCase() == "CHECKBOX")){  
          return false ;  
         }else{  
          return true ;   
         }  
        }else{  
         return false ;  
        }  
       }  
      });