the pep allows for seperators: Pre-release separators Pre-releases should allow a . , - , or _ separator between the release segment and the pre-release segment. The normal form for this is without a separator. This allows versions such as 1.1.a1 or 1.1-a1 which would be normalized to 1.1a1 . It should also allow a seperator to be used between the pre-release signifier and the numeral. This allows versions such as 1.0a.1 which would be normalized to 1.0a1 .