[keycloak-user] Username validation

I'm trying to implement validation on the username. Specifically I'd like to validate a max length and characters that are allowed. I've found some checks in:*RegistrationUserCreation.java and *Validation.java. It looks like the first is used for signups, the latter for changing the account information of existing users. ** How should I go about adding checks? For the Registration I'm guessing that I should copy *RegistrationUserCreation, add the checks, and create a new Registration Flow with the new SPI. Is this correct? But what about the Validation for existing accounts? I could do the same. Make my own copy of Validation.java, but how do I plug it into the system? Any help much appreciated! *