spec: fix regexp

[rogpeppe]

As pointed out by @andaaron in #425, the regular expression proposed there was bogus. It was actually wrong in two different ways. Mea culpa, my apologies.

This fixes it. I wrote a few test cases to check: https://go.dev/play/p/4_iYH4Hao1-

package main

import (
	"regexp"
	"testing"
)

func TestRepoPattern(t *testing.T) {
	r := regexp.MustCompile(`^[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*(/[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*)*$`)
	for _, test := range []struct {
		s    string
		want bool
	}{
		{"foo", true},
		{"foo/bar", true},
		{"foo/bar__baz", true},
		{"foo/bar___baz", false},
		{"/foo", false},
		{"foo//bar", false},
		{"foo-------b__x.com", true},
		{"foo-------b__x.com/p----x", true},
		{"foo-", false},
		{"-foo", false},
		{"foo/-bar", false},
		{"foo/bar-", false},
		{"foo-bar", true},
		{"foo----_bar", false},
		{"foo----bar_/x", false},
	} {
		if got, want := r.MatchString(test.s), test.want; got != want {
			t.Errorf("mismatch on %q; got %v want %v", test.s, got, want)
		}
	}
}

Ideally something like the above would be committed for CI testing but perhaps that's a stage too far.

[imjasonh]

I have no power to approve, but I 100% agree this should be included in code with tests. The regexp doesn't even have to be exported, so long as it's kept in sync with spec.md.

Tests let us catch bugs and regressions, and act as runnable documentation to showcase unusual valid and invalid cases.

[sudo-bmitch]

I'm not sure when it happened, but it looks like we've relaxed the restrictions from the Docker spec:

A repository name is broken up into path components. A component of a repository name must be at least one lowercase, alpha-numeric characters, optionally separated by periods, dashes or underscores. More strictly, it must match the regular expression [a-z0-9]+(?:[._-][a-z0-9]+)*.

https://docs.docker.com/registry/spec/api/