Why does the option --add_extra_groups in adduser contain underscore rather than hyphen word delimiters?

Question

Premise

Most of the long-form options in the various *nix tools use hyphens as word separators.

Examples

from man tr:

 --squeeze-repeats
 --truncate-set1

from man sudo:

--preserve-env
--set-home
--non-interactive

Many more examples could be given.

Question

Why does the option --add_extra_groups in adduser contain underscore rather than hyphen word delimiters? Even the other long-form options in adduser use hyphens:

SYNOPSIS
     adduser  [options] [--no-create-home] [--disabled-password] [--disabled-login] 
     [--add_extra_groups] [--encrypt-home] user

Is this just a typo that has become normalized over time, or is there some other explanation?

Answer 1

It's a typo, even if the option works anyway.

By standard, hyphens should be used to separate words, even if sometimes one can find underscores. For instance, here's what The GNU C Library Reference Manual says about long options:

Long options consist of -- followed by a name made of alphanumeric characters and dashes. Option names are typically one to three words long, with hyphens to separate words.