have you said step-by-step? are you sure?...;=)
Davido's precious regex tester also onther testing site suggested by Eily also, anonimously suggested, rxrx and this one and this other one

Without help i read something like:WRONG match 1,2 or 3 times \w (word char) prefering the minimum amount followed by TGA and capture the result in $1. capture in $1 the last triplet formed of \w followed by TGA

HtH
L*