I'm not using "stupid" in the derogatory sense. I'm using it as a recognition of the skill/knowledge gap between technical and non-technical users.
To clarify, we get _asked_ by our users to implement fields that limit input to help them avoid mistakes. Our QA and UX teams agree. This isn't a unilateral engineering decision.
Yes, you want to provide guidance, but without getting in the way.
That's why I was suggesting letting the user paste in these 'wrong' characters, but offering help in removing them.
Of course, that's a trade-off. And it's more annoying to design and implement than just forbidding those characters from entering the input field in the first place.
To clarify, we get _asked_ by our users to implement fields that limit input to help them avoid mistakes. Our QA and UX teams agree. This isn't a unilateral engineering decision.