A regular diet for rabbits *is* extremely important. Though baby rabbits (Under 6 mos.) Have very different dietary needs from adults.
When they are young, they need a 24/7 supply of good protein rabbit pellets, and a 24/7 supply of alfalfa hay (Or an alfalfa/timothy hay mix, makes the transition from the alfalfa to timothy only easier)
Their green veggies need to be introduced very slowly, one at a time in small quantaties and increased gradually. By the time the rabbits are 6-7 mos. old, their veggies should continue to be increased, their pellets slowly decreased, and their alfalfa hay should decrease from their diet, being replaced with timothy until it's been safely taken out of their diet. By the time a rabbit is 12 mos. old, they should have no alfalfa hay, they should have a large amount varied greens that had been carefully introduced and increased daily, they should have timothy hay 24/7 (Very important) and a very small amount of rabbit pellets, if any. My adult rabbits get a small small bowl of pellets in the morning, their hay is refreshed 3-4 times a day, and they get their veggies every evening. Rabbits have very, very sensitive digestive systems, so it is important to use caution when changing their diet.