The regex is ^[^@\s]+@\S+$

This would reject valid email addresses using quoted parts like john."@".doe@valid.example.

Edit: Looking at it closely it looks like the attempt is to reject anything with an '@' in the user part, which would be an error (e.g. john."@".doe@valid.example). Except since "\S+" also includes '@' this regex will still work, but by accident. It would reject the valid email john."\ ".doe@valid.example