Perhaps it could be turned on and off as needed: lose a finger in an accident, and take a drug to turn regeneration on. Grow your finger back in a week, and then take another drug (or stop taking the first drug) to go back to normal.
Advanced genetic engineering can be used to ensure that particular genes get activated only in the presence of some molecule that isn't naturally present in humans, so it would happen only if you take that particular drug; but it would still be applicable only to "newly built" humans e.g. as part of in vitro fertilization, not as something that can be applied to adults.