Are confederate tattoos against Army regulations?

You should totally go for it! The bigger the better. Of course I am out of the Army so my advice probably shouldn't be followed in this instance.

If you are bent on getting something like that the 1st national flag might be less noticeable. The average person generally isn't that educated to know what it is. They only recognize the battle flag lol. Of course you have the freedom to choose and accept the consequences for your actions. As my old NCO put it "Do what your rank can handle".

If you don't already have it, don't get it. As stated by SFC Kelly Fuerhoff it's a real grey area. Regardless of your intentions it might be perceived as racist. I know that a lot of southerners see it as heritage and not racist, but others might not. Remember that the battle flag of the army of northern Virginia and the Confederate flag are both symbols of a country born out of treason towards the United States. Being a US soldier with that symbol on himself might send signals that you don't respect the country you serve.

Look at it this way. Everything in the Army is perception. Although you might not find is offensive, another might. That being said offensive ink is not in regs. I would advise against it. If you need further guidance and are that hard up for it, speak with your command.