function validateAmount(amount, e){
 if(amount.value.match( /^[0-9]+(\.([0-9]+))?$/)){
  return true;
 }else{
    e.preventDefault();
  alert('You must enter a valid donation.');
  amount.focus();
  return false;
 }
}

document.observe('dom:loaded', function() {
    $('donationform').observe('submit', function (e) {
        validateAmount(this.item_price_1, e)
    });
});