Adding space after set of letters

I usually name my folders like this:

[CAT NR#] Artist - Album

[BRR014] Neurodriver - Battledroid

I'm trying to make it look like this:
[BRR 014] Neurodriver - Battledroid

I'm not really good with regular expressions so I'd appreciate some help.

All right, after couple of hours I managed to do it!

Just looking at it makes me cry blood, but whatever:

$replace(%_directory%,$regexp($left(%_directory%,$strchr(%_directory%,']')),\D,), $regexp($left(%_directory%,$strchr(%_directory%,']')),\D,))

Is the number always three digits? If so, this action should do it.

Action type: Replace with regular expression
Regular expression: ^[(.)(\d{3})](.)$
Replace matches with: [$trim($1) $2] $trim($3)
[ ] case-sensitive comparison

Most of the times yes, but sometimes not. Thanks anyways!