var QTY_ERR = "Please enter a valid quantity, you should only enter numbers.";

function check(a){if(!a.quantity.value.match(/^[0-9]+$/)){alert(QTY_ERR);return false}return true}
function replace_with(b){c=document.getElementById("large_image");a=c.src.replace(/^(.+\?src=\.\.\/)[^\&]+(\&.+)$/,"$1"+b+"$2");c.src=a;window.scroll(0,0)};