def human_to_seconds(string):
    """Convert internal string like 1M, 1Y3M, 3W to seconds.

    :type string: str
    :param string: Interval string like 1M, 1W, 1M3W4h2s...
                   (s => seconds, m => minutes, h => hours, D => days, W => weeks, M => months, Y => Years).

    :rtype: int
    :return: The conversion in seconds of string.
    """
    interval_exc = "Bad interval format for {0}".format(string)

    interval_regex = re.compile("^(?P<value>[0-9]+)(?P<unit>[{0}])".format("".join(interval_dict.keys())))
    seconds = 0

    if string is None:
        return None
    while string:
        match = interval_regex.match(string)
        if match:
            value, unit = int(match.group("value")), match.group("unit")
            if unit in interval_dict:
                seconds += value * interval_dict[unit]
                string = string[match.end():]
            else:
                raise Exception("'{0}' unit not present in {1}".format(
                    unit, interval_dict.keys()))
        else:
            raise Exception(interval_exc)
    return seconds