How are Docker image names parsed?

TL;DR: The hostname must contain a . dns separator, a : port separator, or the value "localhost" before the first /. Otherwise the code assumes you want the default registry, Docker Hub.

---

After some digging through the code, I came across distribution/distribution/reference/reference.go with the following:

// Grammar
//
//  reference                       := name [ ":" tag ] [ "@" digest ]
//  name                            := [hostname '/'] component ['/' component]*
//  hostname                        := hostcomponent ['.' hostcomponent]* [':' port-number]
//  hostcomponent                   := /([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])/
//  port-number                     := /[0-9]+/
//  component                       := alpha-numeric [separator alpha-numeric]*
//  alpha-numeric                   := /[a-z0-9]+/
//  separator                       := /[_.]|__|[-]*/
//
//  tag                             := /[\w][\w.-]{0,127}/
//
//  digest                          := digest-algorithm ":" digest-hex
//  digest-algorithm                := digest-algorithm-component [ digest-algorithm-separator digest-algorithm-component ]
//  digest-algorithm-separator      := /[+.-_]/
//  digest-algorithm-component      := /[A-Za-z][A-Za-z0-9]*/
//  digest-hex                      := /[0-9a-fA-F]{32,}/ ; At least 128 bit digest value

The actual implementation of that is via a regex in distribution/distribution/reference/regexp.go.

But with some digging and poking, I found that there's another check beyond that regex (e.g. you'll get errors with an uppercase hostname if you don't don't include a . or :). And I tracked down the actual split of the name to the following in distribution/distribution/reference/normalize.go:

// splitDockerDomain splits a repository name to domain and remotename string.
// If no valid domain is found, the default domain is used. Repository name
// needs to be already validated before.
func splitDockerDomain(name string) (domain, remainder string) {
    i := strings.IndexRune(name, '/')
    if i == -1 || (!strings.ContainsAny(name[:i], ".:") && name[:i] != "localhost") {
        domain, remainder = defaultDomain, name
    } else {
        domain, remainder = name[:i], name[i+1:]
    }
    if domain == legacyDefaultDomain {
        domain = defaultDomain
    }
    if domain == defaultDomain && !strings.ContainsRune(remainder, '/') {
        remainder = officialRepoName + "/" + remainder
    }
    return
}

The important part of that for me is the check for the ., :, or the hostname localhost before the first / in the first if statement. With it, the hostname is split out from before the first /, and without it, the entire name is passed to the default registry hostname.