Can't create User with some characters in First or Lastname

We discovered that at least the following characters are blocked in the portal and also the REST API
- !
- %

But there is no documentation on this to be found, would greatly appreciate some explanation about why this is and also a correct filter so we can prevent problems like people creating an account like:
'Admin' 'Yahoo!'

which will not succeed when creating a User.

Thanks!

Start by reproducing User creation through the developer portal and REST API with the reported characters ! and %. Trace where the validation behavior is exposed, then document the restriction, its rationale, and a correct filter so clients can prevent rejected names.

Written by the indexing model from the issue text.