What is the standard formatting for RH-SSO username?

Currently, the customer has an existing user management table and uses Custom User Providers [1] to connect to an existing oracle database from RH-SSO

But along the way, the customer realized that some of the usernames cannot be used to login into RHSSO due to some limitation (for example, some existing usernames are case-sensitive, etc)

They are planning to do some fixing in the existing username table, but looking for best practices and standards regarding usernames in RHSSO such as can RHSSO accept alphanumeric, uppercase-lowercase, space, special characters, or any particular requirement as a username. So, I can't find this information in the current documentation, and looking into the code, I found some classes that validate the user, however, this validation seems not to be used.

[1]

https://access.redhat.com/solutions/3221021