User name requirements

User names:

• Must start with a lowercase letter.

• Can contain numbers and lowercase letters.

• Can include only these three special characters: hyphens ( - ), dots ( . ), and underscores ( _ ).

• Can have a maximum of 32 characters.

• Cannot be empty.

• Cannot contain uppercase letters.

• Cannot be: admin, root, or remote_user.

• Cannot be Linux reserved names such as:

  daemon, bin, sys, sync, proxy, www-data, backup, list, irc, gnats, nobody, systemd-bus-proxy, sshd, messagebus, rpc, systemd-journal-gateway, systemd-journal-remote, systemd-journal-upload, systemd-timesync, systemd-coredump, systemd-resolve, rpcuser, vagrant, opsd, rdanet, _lldpd, rdaadmin, rdaweb