I want to create user accounts named after a domain name. adduser complains that the usernames need to match the NAME_REGEX regular expression.

adduser: Please enter a username matching the regular expression configured
via the NAME_REGEX configuration variable.  Use the `--force-badname'
option to relax this check or reconfigure NAME_REGEX.

I can add the users using useradd without complaint. Is there a reason that I shouldn't modify the regular expression to allow ., - and _?

What characters will cause problems and shouldn't be allowed in usernames?

This is the default NAME_REGEX.

  • 2
    Note that NAME_REGEX already accepts - as long as it's not the first character. Sep 3 '15 at 6:02
  • Why not add .? Consider a user named . or ... Then, rm that user named ...
    – Jon
    May 5 '17 at 19:08
  • 2
    @Jon that's not an issue since rm is not the command to use when deleting a user. I agree .. is not a sensible name for similar reasons, but rm is not one of those.
    – toon81
    Oct 29 '18 at 12:14

My advice to you is to follow the standard recommended by the default NAME_REGEX. You can actually put nearly anything in a user name under *NIX but you may encounter odd problems with library code that makes assumptions. Case in point:


My question to you: do you have a lot of domain names that would collide with each other if you stripped out the unusual punctuation? For example, do you have both "QUALITY-ASSURANCE" and QUALITYASSURANCE" as domain names? If not, you could simply adopt a policy of stripping out the unusual characters and using what's left as the user name.

Also, you could use the "real name" section of the GECOS field in the /etc/passwd information to store the original, unmodified domain name, and scripts could extract it pretty easily.

  • It is the running into random unexpected bugs part that I'm worried about. I can pretty easily remove the periods and still have no chance of name clashes, but the - could cause a problem. Still it is pretty unlikely.
    – Ed Haber
    Oct 9 '09 at 23:28
  • So the debian system I'm using is using a user www-data. So it looks like - should be ok to be used in usernames.
    – Ed Haber
    Oct 13 '09 at 0:36
  • Actually, that regular expression permits '-' in user names! The first letter needs to be a-z, but subsequent letters of the user names can be '-', a-z, or 0-9.
    – steveha
    Oct 13 '09 at 18:22
  • Ohh! you're right. I missed the extra - when i was looking at it.
    – Ed Haber
    Oct 14 '09 at 15:12

More specifically, the POSIX ("Portable Operating System Interface for Unix") standard (IEEE Standard 1003.1 2008) states:

3.437 User Name

A string that is used to identify a user; see also User Database. To be portable across systems conforming to POSIX.1-2017, the value is composed of characters from the portable filename character set. The <hyphen-minus> character should not be used as the first character of a portable user name.

3.282 Portable Filename Character Set

The set of characters from which portable filenames are constructed.

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 . _ -

Any username that complies with this standard is POSIX-compliant, and ought to be safe.

  • 7
    While this is true it's generally frowned upon to have upper-case characters in usernames - people have enough trouble with case-sensitive passwords, and making them have to remember case in their usernames is just kicking them when they're down. (Exception: When your username convention is ALL UPPERCASE CHARACTERS.)
    – voretaq7
    Feb 25 '14 at 21:28
  • 2
    As of POSIX.1-2017, those definitions have moved a bit. 3.431 User Name is now 3.437 User Name and 3.278 Portable Filename Character Set is now 3.282 Portable Filename Character Set Sep 14 '18 at 17:50
  • 2
    @voretaq7 What I think is legit is to preserve case in a username, but make sign-ins case-insensitive. So a username could be CatInTheHat but sign in specifying catinthehat or catintheHAT or whatever. Sep 6 '19 at 22:54
  • So, if we have a user FOO and a user foo, and I come around and try to sign in as "Foo"... what happens?
    – Tom Hundt
    May 30 '20 at 1:24
  • That would be an invalid username. May 30 '20 at 15:25

From the NAME_REGEX can be deduced that everything but a through z in upper- and lowercase and the number 0 through 9 would be bad.

  • The word you're looking for is "deduced".
    – wfaulk
    Oct 9 '09 at 20:51
  • 1
    look at the regex again, - is also accepted (just not as the first character.) Also note that by default uppercase is not accepted (there is no g flag, nor is A-Z included in theNAME_REGEX regex provided by the OP. NAME_REGEX="^[a-z][-a-z0-9]*\$" May 10 '18 at 0:03
  • 1
    @SherylHohman You actually meant "there is no i flag...", I suppose.
    – Jens Moser
    Jul 10 '20 at 13:15
  • @JensMoser, lol, yes I did. Thanks. Jul 10 '20 at 16:33

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