kuratowsky

// let MAX_LENGTH_PHONENUMBER = 15
// let ACCEPTABLE_NUMBERS     = "0123456789"

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
                    
            let newLength: Int = textField.text!.count + string.count - range.length
            let numberOnly = NSCharacterSet.init(charactersIn: ACCEPTABLE_NUMBERS).inverted
            let strValid = string.rangeOfCharacter(from: numberOnly) == nil
            return (strValid && (newLength <= MAX_LENGTH_PHONENUMBER))