My given regex archives exactly this :P you can test it with an online regex tester. I dont know if the bot supports this kind of regex.

You could ofc also enforcy only one or max 3 char's between the [ ] and force it to be at the start

^(?!\[.{1,3}\]).*

Maye negating regex work? Something like

^(?!.*\[.+\]).*

(Everything without a [ followed by any character at least one time followed by ] )