How to set validation to allow only alphanumeric characters - no symbols etc

[Daisy_Ramirez]

I have a text field where I’d only want to allow alphanumeric characters - no symbols. I wasn’t able to find anything here on it. I have a field that should contain only 22 characters of which the first two must be “RS” and the last 20 should be numeric.

Can someone assist with how to validate the last 20? Thanks so much