1.0
        1.0.4a3
        1.0.4b1
        1.0.4

    The following are examples of invalid version numbers:

        1
        2.7.2.2
        1.3.a4
        1.3pl1
        1.3c4

    The rationale for this version numbering system will be explained
    in the distutils documentation.
    z)^(\d+) \. (\d+) (\. (\d+))? ([ab](\d+))?$c