//only allow numeric keys
function check_input(e) {
  var keyCode = (e.keyCode) ? e.keyCode : e.which;
    if( (keyCode > 47 && keyCode < 58) || keyCode == 0 || keyCode == 8 ) {
             return true;
    }else{
         return false;
    }
}


function buy_book(form){

  if(form.quantity.value > 0)
    form.submit();

}