Battery drains only when car is running... Help

When I read the OP on this, I too guessed that the regulator might be at fault, but after reading 67Dart's reply, I realized that following the procedure that he outlined made a lot more sense. Doing so would have verified the condition of all the components and given the OP real insight into the operation of the charging system. Anyone can replace parts, it takes a mechanic to diagnose a problem. He gambled his $30 and lucked out. If he had guessed wrong. he'd be out $30 and no closer to a repair.

BTW, just because his immediate problem is solved doesn't mean the alternator's good, it could still have a couple of bad diodes and marginal output.
JMHO