It is not critical, but it is a little frustrating.
When trying to set-up secure user names the only restriction I could find was that "it is recommended that the user name start with a lower case character".
But, when I try to set-up secure user names, which use a mix of lower and upper case characters, numbers and symbols I just get an error that the user name is invalid with the message that "the user name "must" (not should) start with a lower case character". Even after correcting this I got the same error message over and over. So, I had to remove all special characters and just use straight names for Linux to accept the user names.
That is not very secure.
What restrictions are there in setting up user names? What symbols, ASCII characters, are not allowed? I do not see anything specified in the books and forums I have searched.
Is there a restriction in the length?