def get_id_from_tag(tag: str=None) ->str:
    """
    Returns the ID number from a given MangaDex tag.

    Parameters:
        t (str): MangaDex title tag

    Returns:
        str: MangaDex title ID number
    """
    if tag is None or not tag.lower().startswith('mangadex:'):
        return ''
    return tag[len('MangaDex:'):]
