Expires cards in textField

Question

How can I write two characters through "/" in a text box? We need this format "MM / YY", the user will enter just numbers and they are automatically set like this 01/01

override func viewDidLoad() {
        super.viewDidLoad()
 self.expiresTextField.addTarget(self, action: #selector(didChangeText(textField:)), for: .editingChanged)
}

@objc func didChangeText(textField:UITextField) {
        expiresTextField.text = self.expiresString(creditCardExpiresString: textField.text!)
    }

  func expiresString(creditCardExpiresString : String) -> String {

        //here you need to write an implementation

        return  creditCardExpiresString
    }  

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let newLength = (textField.text ?? "").count + string.count - range.length

    if(textField == expiresTextField) {
        return newLength <= 5
    }
    return true
}