def SUB_STR_BYTES(string, index, length):
    """
    Returns the substring of a string.
    The substring starts with the character at the specified UTF-8 byte index (zero-based) in the string
    and continues for the number of bytes specified.
    https://docs.mongodb.com/manual/reference/operator/aggregation/substrBytes/
    for more details
    :param string: The string or expression of string
    :param index: Indicates the starting point of the substring.
    :param length: Can be any valid expression as long as it resolves to a non-negative integer or number
    :return: Aggregation operator
    """
    return {'$substrBytes': [string, index, length]}
