code: def register_validator(flag_name,
                         checker,
                         message='Flag validation failed',
                         flag_values=_flagvalues.FLAGS):
    
    v = SingleFlagValidator(flag_name, checker, message)
    _add_validator(flag_values, v)
docs: Adds a constraint, which will be enforced during program execution.
  
    The constraint is validated when flags are initially parsed, and after each
    change of the corresponding flag's value.
    Args:
      flag_name: str, name of the flag to be checked.
      checker: callable, a function to validate the flag.
          input - A single positional argument: The value of the corresponding
              flag (string, boolean, etc.  This value will be passed to checker
              by the library).
          output - bool, True if validator constraint is satisfied.
              If constraint is not satisfied, it should either return False or
              raise flags.ValidationError(desired_error_message).
      message: str, error text to be shown to the user if checker returns False.
          If checker raises flags.ValidationError, message from the raised
          error will be shown.
      flag_values: flags.FlagValues, optional FlagValues instance to validate
          against.
    Raises:
      AttributeError: Raised when flag_name is not registered as a valid flag
          name.