It was probably built with 5.0, which is what WoW started out with. % used to be the upvalue operator, and in 5.0 it didn't exist at all. In 5.1, it became the modulus operator.
Look for a lua/luac.exe in the program folder and see if you can't update it to 5.1.
If all else fails, math.mod() is a valid solution.
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たしかにひとつのじだいがおわるのお
ぼくはこのめでみたよ
だけどつぎがじぶんおばんだってことわ
しりたくなかったんだ
It's my turn next.
Shakespeare liked regexes too!
/(bb|[^b]{2})/
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