how to validate data in array in CI4?

Hello,
i have a restfull api controller in ci4.
I validate the parameter of the request.
works fine.
But one parameter is a comma seperated list of domainnames.
To make things worse, some times the liste contains space, tab or linefeed as seperator.


I need to verify each name against the db to ensure it is not allready there.
How do I run validate against an array of domainnames or a single domainname within a loop?
I found some samples with ci3 which do not work.
The docu does not show anything about this.
Thanks
Stefan