In the Admin Console you can add users to a File Realm in the following screen:
Configuration> Security> Realms> file "New File Realm User"
The User ID has the following comment:
Name of a user to be granted access to this realm; name can be up to
255 characters, must contain only alphanumeric, underscore, dash, or dot
If I include other symbols ('+', etc.) I get an error message ("Invalid
characters in name: abc+3.")
It is my understanding that "alphanumeric" does not include multibyte
(like Japanese) characters. But when I include Japanese characters in the User
ID, they are accepted without error.
If multibyte characters are supported the comment should be improved. If the
comment is correct and they are not supported, the validation logic could be