Sorry, missed you topic. Encase this happens please PM the link to your topic if it has been more than 3 days.

Regarding your needs. I suggest creating a Text field that's set to Read Only. Next use CB Auto Actions using a Query action on the after registration trigger. Next you can perform a database query to force a new value to the field using SQL functions to format it to your needs.

For your particular usecase there isn't a tutorial, but there's a few usage tutorials below for CB Auto Actions. There isn't any documentation however as there isn't too much to document as it's a tool and doesn't do anything without specific configurations.

www.joomlapolis.com/support/tutorials/107-use-cases

The below tutorial is probably closest to your usage as it updates a _comprofiler column in one of its actions.

www.joomlapolis.com/support/tutorials/107-use-cases/18219-cb-auto-actions-changing-approval-workflow-based-on-cb-field-value

As for SQL I suggest the below tutorial to get started.

www.w3schools.com/sql/default.asp

To generate a unique string in SQL you'd use UUID(), but it's not going to be in the format you're wanting.

dev.mysql.com/doc/refman/5.6/en/miscellaneous-functions.html#function_uuid

Regarding the format you're wanting I don't really see that being unique enough for 100's of thousands of entries as it's not long enough for that; so if it's possible you may want to just keep the output UUID provides.

The alternative to use SQL is to use a Code action and set its Method to PHP. You could then use PHP to generate your unique id, run database queries to make it isn't used, then update the user object using API. To update a CB user object using API please see the below tutorial.

www.joomlapolis.com/support/tutorials/120-api-usage/18362-registering-a-user-through-cb-api

See the "editUser" code block at the bottom for an example.