Select string between parenthesis

Hello

I'm asking something which is probably pretty easy, but I can't find a way with the string functions (probably because I didn't understand how the $str*** functions work).

I want to select just the string between two square parenthesis.

As example, in:
1983 - Kill 'Em All [25DP 5339]
I just want [25DP 5339] (possibly with the parenthesis, but everything will work.

How can I do that?

assuming that the string is in the ALBUM tag field:

$regexp(%album%,'.['(.+)'].',$1)

or easier, but not usable as a scripting function which you can built into other actions:
assuming that you want the string into the CATALOG # tag field.

Action: Guess Value
Sourceformat: %album%
Guessing Pattern: %dummy% [%catalog #%]