:returns: A rational version string, or None, if couldn't determine one.

    """

    try:

        NormalizedVersion(s)

        return s   # already rational

    except IrrationalVersionError:

        pass

    rs = s.lower()

    # part of this could use maketrans

    for orig, repl in (('-alpha', 'a'), ('-beta', 'b'), ('alpha', 'a'),

                       ('beta', 'b'), ('rc', 'c'), ('-final', ''),

                       ('-pre', 'c'),

                       ('-release', ''), ('.release', ''), ('-stable', ''),

                       ('+', '.'), ('_', '.'), (' ', ''), ('.final', ''),

                       ('final', '')):

        rs = rs.replace(orig, repl)

    # if something ends with dev or pre, we add a 0

    rs = re.sub(r"pre$", r"pre0", rs)

    rs = re.sub(r"dev$", r"dev0", rs)

    # if we have something like "b-2" or "a.2" at the end of the

    # let's remove the dash or dot

    rs = re.sub(r"([abc|rc])[\-\.](\d+)$", r"\1\2", rs)

    # 1.0-dev-r371 -> 1.0.dev371

    # 0.1-dev-r79 -> 0.1.dev79

    rs = re.sub(r"[\-\.](dev)[\-\.]?r?(\d+)$", r".\1\2", rs)

    # Clean: 2.0.a.3, 2.0.b1, 0.9.0~c1

    rs = re.sub(r"[.~]?([abc])\.?", r"\1", rs)

    # Clean: v0.3, v1.0

    if rs.startswith('v'):

        rs = rs[1:]

    # Clean leading '0's on numbers.

    #TODO: unintended side-effect on, e.g., "2003.05.09"

    # PyPI stats: 77 (~2%) better

    rs = re.sub(r"\b0+(\d+)(?!\d)", r"\1", rs)

    # Clean a/b/c with no version. E.g. "1.0a" -> "1.0a0". Setuptools infers

    # zero.

    # PyPI stats: 245 (7.56%) better

    rs = re.sub(r"(\d+[abc])$", r"\g<1>0", rs)