Can water siphon backwards through the main pressure regulator?
If you have a backflow preventer, aka a check valve, installed in the system, then the water will not siphon back. This could be installed near the pressure regulator or main shutoff inside your home. It's also possible for the municipality to install one near the meter.

A standard pressure reducing valve closes when the water pressure inside reaches a threshold. Once the inside pressure drops slightly, the valve opens, at which point a backflow is possible.