A missing END for a LOOP instruction gives the following error:
26 - return groups == 0
Error 14 running C:\ORexxDev\Rosetta Code\oorexx\balancedbrackets.rex line 26:
Error 14.1: DO instruction on line 17 requires matching END
Two problems: 1) The primary message should also include LOOP.
2) There probably needs to be a version of the secondary message for LOOP.