function data_change(field)
     {
          var check = true;
          var value = field.value; //get characters
          //check that all characters are digits, ., -, or ""
          for(var i=0;i < field.value.length; ++i)
          {
               var new_key = value.charAt(i); //cycle through characters
               if(((new_key < "0") || (new_key > "9")) && 
                    !(new_key == ""))
               {
                    check = false;
                    break;
               }
          }
          //apply appropriate colour based on value
          if(!check)
          {
          	 a=value.substr(0,field.value.length-1);
          	 field.value =a;
             
          }
          else
          {
               field.style.backgroundColor = "white";
          }
     }