diff --git a/specification/json/cper-json-header.json b/specification/json/cper-json-header.json
index 2980dc0..8edb4e1 100644
--- a/specification/json/cper-json-header.json
+++ b/specification/json/cper-json-header.json
@@ -103,7 +103,7 @@
             "properties": {
                 "guid": {
                     "type": "string",
-                    "pattern": "^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{16}$"
+                    "pattern": "^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
                 },
                 "type": {
                     "type": "string"