No explanation on invalid characters on registration form

[molnarm]

When a user tries to register with a username containing special characters (e.g. accented letters), WP shows an error message saying "This username is invalid because it uses illegal characters. Please enter a valid username.".

However, this doesn't really help the user because the message does not explain how to fix the problem, i.e. which characters are allowed.

Please consider adding a brief listing of allowed characters.

Related: [14428], #13162

Patch adds a message modelled on what Google tells you when you try to sign up for an account using funky characters.

I should say that sanitize_user() also allows spaces and @ in usernames, but I'm not sure we should be encouraging that?

same just refreshed