Extract catalogue number from album field

Hi, can someone who's good at this help? i'm not sure i have the brain for this coding but it would save me so much time. I have a lot of tags with a catalogue number in between square brackets amongst other stuff. How do i removed everything except the catalogue number. Here's an example: [BT 03] - 1995 - Spikey Trance. I just want it to say BT 03

is that possible??

Which tag field does contain this data?
You can use an action of the type "Guess value" (import tag fields)
Source: %catalogue number%
(please insert here the correct name for the tag field)
Target:[%catalogue number%] %dummy%
(here again: please insert the correct name for the tag field)

ah, ok i'll try that tonight, it's in album at the moment. I see, so i replace %dummy% with %album% and yes my fields exact target field name is %catalog #%

It the data

can be found in ALBUM, then the it is
Source: %album%
Target: [%catalog #%] - %album%
and probably
Target: [%catalog #%] - %year% - %album%

Ah, actually i don't think that is what im trying to do. Right now, the album field already says all of this " [BT 03] - 1995 - Spikey Trance" and the catalog # tag is blank. I want catalog # to say "BT 03"

My brother suggested maybe first, simply copy album field to catalog #, then use something like

$regexp(%catalog #%, "^\.*[(.*?)\].*$", $1)
on the catalog # field to keep only whats inside the brackets
$regexp(%album%, "[(.*?)\] ?", "")
on the album field to remove the brackets and everything inside them (as i don't want the catalog number in album field once its copied over to the catalog # field

perhaps that code needs correcting. I don't know if there's a better way

2nd try
$regexp(%catalog%, ^. [(. ?)].*$, $1)
Think should give you just what's in the brackets from catalog

$regexp(%album%, [(.*?)] ?, );
should replace [blahblah123] with empty strings

And all this can be achieved in one go (without the 2 steps of copying first and then removing) with that what I described in

Ah sorry yes that totally works. Brilliant! I think i understand what the code is saying too (a bit) now.

Just need 1 extra % sign [%catalog #%] - %year% - %album%