At present, when given a CQL query like
mod-users matches any username that contains the sequence "riv". I guess under the hood it's doing something like USERNAME LIKE '*riv*'.
But that's not right. CQL is word-oriented. username=riv should match only the username riv; username=riv* should match all usernames beginning with riv; and of course username=*riv* should match all usernames with an riv anywhere in them.