Basically it has to be the same configuration of the factory exhaust ie: if the car came out with 3 mufflers, the aftermarket exhaust has to have 3 mufflers. It has to pick up on all original hangers, it has to exit at least 50mm past the last welded body seam, it can't protude past the position of where the original tip finished and it can't be over 86dB. There is an obscure rule, 312 I think, that says ANY exhaust modification requires an engineers certificate, but it is very rarely enforced. Most exhaust regulations are standardized across the country, but it will always come down to a police officer's opinion on what he deems illegal. It is up to you to get it inspected and passed if you get defected.
its real simple... if its loud then you gotta be careful where you open it up... i got mine done so that below 2500rpm its louder than stock but not enough to really create attention, once it gets over that though it gets a lot louder than stock... pretty much if your acting the idiot a cop is gonna defect you anyway he can