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//when key is pressed in the textbox
$("#quantity").keypress(function (e)
{
  //if the letter is not digit then display error and don't type anything
  if( e.which!=8 && e.which!=0 && (e.which<48 || e.which>57))
  {
    //display error message
    $("#errmsg").html("Digits Only").show().fadeOut("slow");
    return false;
  }
});

//when key is pressed in the textbox
$("#quantity").keypress
(
    function (e) {
        //if the letter is not digit then display error and don't type anything
        if( e.which!=8 && e.which!=0 && (e.which<48 || e.which>57)) {
            //display error message
            $("#errmsg").html("Digits Only").show().fadeOut("slow");
            return false;
        }
    }
);