const (
	defaultNamespace = "library"
	repositoryChars  = "abcdefghijklmnopqrstuvwxyz0123456789_-./"
	regRepoDelimiter = "/"
)

...

	if err := checkRepository(repo); err != nil {
		return Repository{}, err
	}

...

func checkRepository(repository string) error {
	return checkElement("repository", repository, repositoryChars, 2, 255)
}

...

func checkElement(name, element, allowedRunes string, minRunes, maxRunes int) error {
	numRunes := utf8.RuneCountInString(element)
	if (numRunes < minRunes) || (maxRunes < numRunes) {
		return newErrBadName("%s must be between %d and %d characters in length: %s", name, minRunes, maxRunes, element)
	} else if len(strings.Map(stripRunesFn(allowedRunes), element)) != 0 {
		return newErrBadName("%s can only contain the characters `%s`: %s", name, allowedRunes, element)
	}
	return nil
}