#3191 introduces a regex to validate what valid tags are. This led to a validation failure for localhost:5000/example:latest, and according to the description in https://docs.docker.com/reference/cli/docker/image/tag/#description should be allowed. It has practically worked before as well, so I think this is too narrow validation.

There is an example in docker docs that also fails validation: myregistryhost:5000/fedora/httpd:version1.0