function GetCountryRegex() {
  if (scope.countryCode == "DE") {
    return '/^(?!01000|99999)(0[1-9]d{3}|[1-9]d{4})$/';
  }
  if (scope.countryCode == "AT") {
    return '/^\d{4}$/';
  }
}

var regExp = new RegExp(GetCountryRegex());
regExp.test(...);