run

var (
	// ErrReferenceInvalidFormat represents an error while trying to parse a
	// string as a reference.
	ErrReferenceInvalidFormat = errors.New("invalid reference format")
	// ErrTagInvalidFormat represents an error while trying to parse a string as a
	// tag.
	ErrTagInvalidFormat = errors.New("invalid tag format")
	// ErrDigestInvalidFormat represents an error while trying to parse a string
	// as a tag.
	ErrDigestInvalidFormat = errors.New("invalid digest format")
	// ErrNameEmpty is returned for empty, invalid repository names.
	ErrNameEmpty = errors.New("repository name must have at least one component")
	// ErrNameTooLong is returned when a repository name is longer than
	// NameTotalLengthMax.
	ErrNameTooLong = fmt.Errorf("repository name must not be more than %v characters", NameTotalLengthMax)
)

var (

	// TagRegexp matches valid tag names. From docker/docker:graph/tags.go.
	TagRegexp = match(`[\w][\w.-]{0,127}`)

	// DigestRegexp matches valid digests.
	DigestRegexp = match(`[A-Za-z][A-Za-z0-9]*(?:[-_+.][A-Za-z][A-Za-z0-9]*)*[:][[:xdigit:]]{32,}`)

	// RuntimeRegexp
	RuntimeRexp = expression(
		optional(capture(alphaRegexp), literal(`://`)))

	// NameRegexp is the format for the name component of references. The
	// regexp has capturing groups for the hostname and name part omitting
	// the separating forward slash from either.
	NameRegexp = expression(
		optional(hostnameRegexp, literal(`/`)),
		nameComponentRegexp,
		optional(repeated(literal(`/`), nameComponentRegexp)))

	// ReferenceRegexp is the full supported format of a reference. The regexp
	// is anchored and has capturing groups for name, tag, and digest
	// components.
	ReferenceRegexp = anchored(capture(RuntimeRexp),
		capture(NameRegexp),
		optional(literal(":"), capture(TagRegexp)),
		optional(literal("@"), capture(DigestRegexp)))
)