AC/DC Voltage Regulator with Floating Input

In looking at several AC to DC regulator circuits, I've notice that if one wants a positive output voltage, say 5V, one could obviously use a +5V regulator or use a -5V regulator while tying the output to ground. In the first case (+5V reg) the input AC supply is attached to the same ground as the regulator. But with the -5V regulator to supply the +5V, the AC supply is left floating or isolated from the regulator.

My question is what is the advantage of using the negative regulator for a positive output with the AC supply floating? Does it supply some sort of protection for what is downstream of the regulator?

Thanks

the input AC supply is attached to the same ground as the regulator

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Are you talking about the ground for the mains supply?

Post a schematic of what you are referring to.

A voltage regulator needs a DC input, not an AC input. The AC from a transformer is rectified into pulsing DC then a huge filter capacitor smooths the pulses into fairly steady DC.

Thank you for your responses. Here are the examples I found.

Yes, the AC would first need to be rectified and smoothed before connecting to the regulator. I'm just wondering at that point what is the advantage and disadvantage of the attached circuits are.

The advantage is that one type of regulator can be used for two purposes, simplifying sourcing.