How to use Validation: Contains

I can't figure out how to use the validation "Contains" field. I want users to be able to enter numbers and letters only (No space or special characters), but I'm not sure what language/code should I enter here for that purpose.
Thanks

Thank you. That worked.
However, for my knowledge, what is the use for "contain" and how do you use it?

To make sure the contains a specific string

Thank you.