No rule for this one
No ending or word group predicts the article of Regal. Learn it as a pair and say it out loud: das Regal.
das Regal
It's das Regal. Regal is neuter, plural die Regale.
Das Regal ist voll.
The shelf is full.
Die Milch ist im Regal.
The milk is on the shelf.
No ending or word group predicts the article of Regal. Learn it as a pair and say it out loud: das Regal.