regexp for correct package version

[Aleksey Cheusov <cheusov%tut.by@localhost>]

There are a number of packages in pkgsrc tree having version
that doesn't conform to the following regexp

   [0-9]+([.][0-9]+)*(nb[0-9]+)?

Is there a "policy" for this and checks in pkglint?

I need an algorithm of comparing package's versions.
Algorithm, not a tool.

Examples:

? mplayer 1.0rc9nb9 1.0rc10nb1
? GConf 2.18.0.1nb1 2.20.1
? libgnome 2.18.0nb3 2.20.1.1
? mplayer-share 1.0rc9 1.0rc10
? mencoder 1.0rc9nb5 1.0rc10
? libtheora 1.0alpha7 1.0beta2

-- 
Best regards, Aleksey Cheusov.