How to validate comment fields

I need a way to validate fields, that represent comments, to not include certain characters, but the new validation tool is not very intuitive. Everything I have tried has resulted in all existing fields being marked as invalid, even though they should technically be fine. I get how the new system can work for things like asset names, shot names etc, but it seems to fall down at comments. I think this is due to comments not being fixed values. Has anyone found a way to do this? Any help is appreciated. Thanks