check if the format of number is (num)(G|m|B) i.e 500GB, 200mb. 400
        etc..

def get_valid_byte(num, default):
        """check if the format of number is (num)(G|m|B) i.e 500GB, 200mb. 400
        etc.. """
        num_valid = re.match(r"\A([0-9]+)(M|G|m|g|)(B|b|\b)\Z", num, re.I)
        if num_valid:
            return num
        return default