+
+// CollectionIDFromDNSName returns a UUID or PDH if s begins with a
+// UUID or URL-encoded PDH; otherwise "".
+func CollectionIDFromDNSName(s string) string {
+ // Strip domain.
+ if i := strings.IndexRune(s, '.'); i >= 0 {
+ s = s[:i]
+ }
+ // Names like {uuid}--collections.example.com serve the same
+ // purpose as {uuid}.collections.example.com but can reduce
+ // cost/effort of using [additional] wildcard certificates.
+ if i := strings.Index(s, "--"); i >= 0 {
+ s = s[:i]
+ }
+ if UUIDMatch(s) {
+ return s
+ }
+ if pdh := strings.Replace(s, "-", "+", 1); PDHMatch(pdh) {
+ return pdh
+ }
+ return ""
+}