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

You need regex, not "contains".

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/^[A-Za-z0-9]*$/

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.