Clarify requirements for identifiers validation

Fix commentary to be clear that the RFC1035 2.3.1 rules are not
explicitly enforced by the identifiers regex.

Add a test to show the lessened restrictions for digits to follow hyphen
and period.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

identifiers/validate.go

@@ -42,13 +42,13 @@ var (
 	identifierRe = regexp.MustCompile(reAnchor(alphanum + reGroup(separators+reGroup(alphanum)) + "*"))
 )
 
-// Validate return nil if the string s is a valid identifier.
+// Validate returns nil if the string s is a valid identifier.
 //
-// identifiers must be valid domain names according to RFC 1035, section 2.3.1.  To
-// enforce case insensitivity, all characters must be lower case.
+// identifiers are similar to the domain name rules according to RFC 1035, section 2.3.1. However
+// rules in this package are relaxed to allow numerals to follow period (".") and mixed case is
+// allowed.
 //
-// In general, identifiers that pass this validation, should be safe for use as
-// a domain names or filesystem path component.
+// In general identifiers that pass this validation should be safe for use as filesystem path components.
 func Validate(s string) error {
 	if len(s) == 0 {
 		return errors.Wrapf(errdefs.ErrInvalidArgument, "identifier must not be empty")

identifiers/validate_test.go

@@ -34,6 +34,7 @@ func TestValidIdentifiers(t *testing.T) {
 		"swarmkit.docker.io",
 		"0912341234",
 		"task.0.0123456789",
+		"container.system-75-f19a.00",
 		"underscores_are_allowed",
 		strings.Repeat("a", maxLength),
 	} {