username stuff

[Christopher McKinley <lamfada@lugh.net>]

This may be a stupid question, but I need to find a spec on usernames
under linux, and the man pages, HOWTOs, the O'Reilly Nutshell book, and
the assorted other docs on my system seem to not mention what forbidden
characters and whatnot there is, although I remember seeing it at some
point.  This is what I know for sure:

1) username must be 1-8 chars
2) The first char must be a letter
3) none of: ':','=',','
4) no control characters

What other chars are verboten?
Are there any other restrictions?

Thanks in advance...
-Chris