def check_format(input_guess):
    """
    check the format in new guess input and return in upper case.
    :param input_guess: str, the given alphabet of new guess.
    :return: str, the guess in legal format and upper case.
    """
    while True:
        if input_guess.isalpha() and len(input_guess) == 1:
            input_guess = input_guess.upper()
            return input_guess
        input_guess = input("illegal format.\nYour guess: ")