Split page_artid into page_start/end and artid.

def split_page_artid(page_artid):
    """Split page_artid into page_start/end and artid."""
    page_start = None
    page_end = None
    artid = None

    if not page_artid:
        return None, None, None

    # normalize unicode dashes
    page_artid = unidecode(six.text_type(page_artid))

    if '-' in page_artid:
        # if it has a dash it's a page range
        page_range = page_artid.replace('--', '-').split('-')
        if len(page_range) == 2:
            page_start, page_end = page_range
        else:
            artid = page_artid
    elif _RE_2_CHARS.search(page_artid):
        # if it has 2 or more letters it's an article ID
        artid = page_artid
    elif len(_RE_CHAR.sub('', page_artid)) >= 5:
        # if there are more than 5 digits it's an article ID
        artid = page_artid
    else:
        if artid is None:
            artid = page_artid
        if page_start is None:
            page_start = page_artid

    return page_start, page_end, artid