According to this line, gem names are required to have letters. However, the regular expression on the line above only enforces that the gem name contains some non-digit character. That means '2.2' is a valid gem name, but it contains no letters. I'm not sure what the rule is supposed to be, but I thought I'd file a ticket.
I've got this with @sandersch
Requires gem names to have at least one alpha character
@adkron and @sandersch