[keycloak-user] Check to see if user exists with Admin REST API

I hope this is my last question to this list :-) I want to check if a user exists with given username/or email address from within an Android App while the user is typing. Hence I want to provide a REST endpoint to let the app check. I am using the Admin REST API for backchannel requests to keycloak from within my servlet. /admin/realms/{my-realm}/users/{username} (or the get(username) equivalent of admin-client) does not work for checking if the email address is already in use. /admin/realms/{my-realm}/users?search={username} works for email, but it's greedy. I think "%" is added at beginning and end of the query parameters before the database request, so that search hits are quasi guaranteed. E.g. if someone has a registered Email Address of john.smith(a)example.com and someone tries to register with smith(a)example.com or smith(a)example.com.br it is shown as already in use. Any ideas? How is this solved in the web frontend? Maybe the Admin REST API endpoint should support username and email as path params when those two can be used to login? Best Regards Benjamin