> It is usually recommended to only use usernames that begin with > a lower case letter or an underscore, and are only followed by > lower case letters, underscores, dashes, and dollar signs. In > regular expression terms: '[a-z_][a-z0-9_-]*[$]'. > On Debian, the constraints on the username are loosened: Usernames > must neither start by a dash ('-') nor contain a colon (':') or an > end of line ('\n').
Maybe also mention that the dollar sign is only tolerate as the last character of the username (motivated by Samba, actually).
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