I do this kind of renaming all the time, w/o using regular expressions (which I have never fully understood).
If the original filename structure is 1343-4423-122(Some Text Here).pdf and only the (variable length, I assume) "(Some Text Here)" is to be removed, I simply Remove from 14 to 99 (or some other number larger than the length of the longest text). This leaves the 1st 13 characters and removes everything following.
That's one of the great things that I like about BRU . . . often complex tasks can be accomplished using its simplest features. If I don't understand a feature (like regular expressions), I can most often figure a "work around", even if it means multiple steps. Better to do what I understand than create havoc for myself by making mistakes.