function validateForm(form) {

	if (form.zip.value === "")
	{
		alert("Please enter a valid zip code.");
		return false;
	}

	if (form.zip.value.length != 5) {
		alert("A valid zip code has 5 digits.");
		return false;
	}

	for (var i=0; i < form.zip.value.length; i++) {
		var digit = form.zip.value.charAt(i);
		if (digit < "0" || digit > "9") {
			alert("A valid zip code has only digits.");
			return false;
		}
	}

    	return true;
}