@classmethod
def from_str(cls, value):
    """Constructs a Version from a string.
        Returns None if string not in N.N form where N represents a
        number.
        """
    m = cls.VERS_STR_REGEX.match(value)
    if m is not None:
        return cls(int(m.group(1)), int(m.group(2)))
    else:
        return None
