POSIX states this about usernames:

[...] To be portable across systems conforming to IEEE Std 1003.1-2001, the value is composed of characters from the portable filename character set. The hyphen should not be used as the first character of a portable user name.

... where the portable filename character set is:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

a b c d e f g h i j k l m n o p q r s t u v w x y z

0 1 2 3 4 5 6 7 8 9 . _ -

Also, the manpage for the /etc/adduser.conf file states:

   VALID NAMES

          adduser and addgroup enforce conformity to IEEE Std 1003.1-2001,

          which  allows  only  the following characters to appear in group

          and user names: letters, digits, underscores, periods, at  signs

          (@) and dashes. The name may not start with a dash. The "$" sign

          is allowed at the end of usernames (to conform to samba).



          An additional  check  can  be  adjusted  via  the  configuration

          parameter NAME_REGEX to enforce a local policy.

However,

Whilst both specifications seem to include the dot, Ubuntu (on my 13.04 at least) seems to disallow it:

⊳ sudo adduser as.df

adduser: Please enter a username matching the regular expression configured

via the NAME_REGEX[_SYSTEM] configuration variable.  Use the `--force-badname'

option to relax this check or reconfigure NAME_REGEX.

The default NAME_REGEX in Ubuntu is (from the /etc/adduser.conf manpage):

^[a-z][-a-z0-9]*$

• Starting with a lowercase letter then any number of dashes, lowercase letters or digits. No _, @ or ..

So,

in conclusion a dot . may be used for a Ubuntu username, the NAME_REGEX just has to be changed in /etc/adduser.conf. Seeing as it conforms to POSIX, there shouldn't be any problems with having a . in the username with any POSIX-compliant program.

To enable a dot in usernames

1. Run this command in a terminal:

   
   
   

   sudo nano /etc/adduser.conf
   
   




2. Locate this line (near the end of the file)

   
   
   

   #NAME_REGEX="^[a-z][-a-z0-9]*$"
   
   

   
   
   

   and replace it with

   
   
   

   NAME_REGEX='^[a-z][-.a-z0-9]*$'
   
   

   
   
   

   Note that the - must remain the first character in the bracket expression [...], otherwise it is treated as specifying a range a-z.




3. Press Ctrl+X, then Y, then Enter.

References:

• Why are underscores not allowed in usernames in some distros (Debian for example)


• man adduser.conf