diff --git a/docs/sources/api/registry_api.rst b/docs/sources/api/registry_api.rst
index f33ca187bb..7034d90e27 100644
--- a/docs/sources/api/registry_api.rst
+++ b/docs/sources/api/registry_api.rst
@@ -301,7 +301,7 @@ POST /v1/users
     {"email": "sam@dotcloud.com", "password": "toto42", "username": "foobar"'}
 
 **Validation**:
-    - **username** : min 4 character, max 30 characters, all lowercase no special characters.
+    - **username** : min 4 character, max 30 characters, must match the regular expression [a-z0-9_].
     - **password**: min 5 characters
 
 **Valid**: return HTTP 200
@@ -345,6 +345,11 @@ GET /v1/users
 
 The Registry does not know anything about users. Even though repositories are under usernames, it’s just a namespace for the registry. Allowing us to implement organizations or different namespaces per user later, without modifying the Registry’s API.
 
+The following naming restrictions apply:
+
+- Namespaces must match the same regular expression as usernames (See 4.2.1.)
+- Repository names must match the regular expression [a-zA-Z0-9-_.]
+
 4.3.1 Get all tags
 ^^^^^^^^^^^^^^^^^^