[fix] precise USER_ID format

rfc/0009_Duniter_Blockchain_Protocol_V11.md

@@ -128,7 +128,10 @@ Issuing an identity is the act of creating a link between a *public key* and *an
 > « This identity refers to me ! »
 
 #### Identity unique ID
-UCP does not rely on any particular identity format, which remains implementation free. Identity simply has to be a string of length between 2 and 100 characters avoiding usage of line ending characters.
+
+Identity simply has to be a string of length between 2 and 100 characters allowing only alphanumeric characters and `-` and `_`.
+
+The exact regex is `[A-Za-z0-9_-]{2,100}`.
 
 In this document *identifier*, `UserID`, `USER_ID` and `uid` will be indifferently used to refer to this identity string.