$.validator.addMethod(
    "checkZip",
    function(value, element, params) {
        var required = typeof(params.required) != 'undefined' && params.required;
        var regexp = required ? /^\d{5}(?:\-\d{4})?$/ : /^(\d{5}(?:\-\d{4})?)?$/;
        return !$(element).is(":visible") || regexp.test(value);
    },
    "Please enter a valid zip"
);