IMHO:
[AZ_] must be [A-Z_]
the third [AZ_] must be [A-Z0-9_]
I've found:

s/^\[([A-Z_]+)\s([A-Z_]+)\s([A-Z0-9]+)\/([0-9]+)\]$/$1_$2_$3_$4/g
[download]

ok? :-) It works but it's not an inline-change on $3!