I know that some types of worms can generate a new head and/or tail if bisected. But does this always happen, or only under special circumstances?

If I’m digging in my garden and cut an earthworm in half, what are the chances that I’ll end up with two worms, and how long will it take?

If you chop off the 7 or 8 segments of the head end, it might regenerate the remainder (assuming it doesn’t die from the stress of being chopped in half). The tail end will just die; it won’t regenerate the head end.