jQuery.validator.addMethod("zipcode", function(zip) {
zip = zip.replace(/^\s+/, "");
zip = zip.replace(/\s+$/, "");

if(zip.length == 0) {
return true;
}

if(zip.match(/^\d{5}([- ]?\d{4})?$/)) {
return true;
}
return false;
}, "Please specify a valid US ZIP code");