js屏蔽退格键

		function pingbituige(){
			    var code;  
			    if (!e) var e = window.event;  
			    if (e.keyCode) code = e.keyCode;  
			    else if (e.which) code = e.which;  
			    if (((event.keyCode == 8) &&                                                    //BackSpace   
			         ((event.srcElement.type != "text" &&   
			         event.srcElement.type != "textarea" &&   
			         event.srcElement.type != "password") ||   
			         event.srcElement.readOnly == true)) ||   
			        ((event.ctrlKey) && ((event.keyCode == 78) || (event.keyCode == 82)) ) ||    //CtrlN,CtrlR   
			        (event.keyCode == 116) ) {                                                   //F5   
			        event.keyCode = 0;   
			        event.returnValue = false;   
			    }  
			    return true;  
			  }