ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits. If the function is passed a valid PIN string, return true, else return false. Examples (Input --> Output) "1234" --> true "12345" --> false "a234" --> false

function validatePIN (pin) {
  var pin_regex = /^\d{4}$|^\d{6}$/;
  return pin_regex.test(pin);
}

// or 

function validatePIN (pin) {
  return /^(\d{4}|\d{6})$/.test(pin)
}