lightweight, client-side, contact data validation

document.observe('dom:loaded', function() {
      var products = $$('.product');
      var updateTotal = function() {
        var total = 0;
        products.each(function(product) {
          var amount = product.down('.amount');
          if (!amount.value.blank() && ~~(amount.value) > 0) {
            total += parseFloat(amount.value) * parseFloat(product.down('.entry-price').value);
          }
        });
        $('eur').update('EUR ' + total.toFixed(2));
      };
      products.each(function(product) {
        var input = product.down('.amount')
        input.observe('keyup', function(event) {
          if (!input.value.blank() && ~~(input.value) > 0) {
            updateTotal();
          }
        });
      });
    });
    $('shop_form').observe('submit', function(event) {
      event.stop();
      return false;
    })
    $('order_button').observe('click', function() {
      if ($('order_button').hasClassName('disabled')) {
        return;
      }
      var email = $('field_email');
      email.next('.error')[email.value.blank() || /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/.test(email.value) === false ? 'show' : 'hide']();
      $w('name tel address plz town').each(function(identifier) {
        var field = $('field_' + identifier);
        field.next('.error')[field.value.blank() ? 'show' : 'hide']();
      });
      if ($$('.error').all(function(error) { return !error.visible(); }) === true) {
        $('shop_form').submit();
      }
    });
    var agbs = $('agbs_checkbox');
    agbs.observe('click', function() {
      $('order_button')[agbs.checked ? 'removeClassName' : 'addClassName']('disabled');
    });
    $('agbs_link').observe('click', function(event) {
      event.stop();
      window.open('/agbs.html', "AGBs", "width=500,height=500,scrollbars=yes,status=no,toolbar=no,menubar=no");
    });