Docker hub repository name does not except "--" (two dashes)

Attempted to create a docker repository named “some–thing” and syntax checker failed the name. A single dash (“some-thing”) did work. Bug?

The hub message isn’t as comprehensive as it probably could be, but I did notice that this also seems to be a restriction in the docker engine’s regex:

$ docker tag busybox foo--bar
repository name component must match "[a-z0-9]+(?:[._-][a-z0-9]+)*"