Will it damage my 2012 RDX to use regular gas?

If it recommends 91 or higher that's what I would use. Higher octane fuel is for vehicles with a 9.5:1 compression ratio or greater. Failure to use the higher octane gas will cause the gas to ignite in the cylinder before the piston makes it all the way to the top of its stroke. This will seriously effect performance and fuel mileage.

http://auto.howstuffworks.com/fuel-efficiency/fuel- consumption/question90.htm --- check this out

You paid a lot of money for a vehicle performance. Now step up to the plate and put performance fuel in it. Penny wise. Pound foolish. B. Franklin.