I have a config file I'm trying to edit in a script, there are several lines, but the format is basically:

```
...
admin_users = alice, bob
admin_groups = accounting, finance
...

```

I would like to append a user to the admin\_users line if they don't already exist on that line.

As I understand it, I'll need to basically combine two conditionals...

`/^admin_users/s/$/, charlie/` = Add `, charlie` to lines beginning with `admin_users`
`/charlie/!s/$/, charlie/` = Append `, charlie` to lines not containing `charlie`

I've tried googling around but I can't find anything relevant. If this is possible, the syntax and then a short explanation would be greatly appreciated.