I don't see why it wouldn't be. Pex is allowed inside the house, so there's no reason it wouldn't be allowed outside given that it's protected from UV.
Around here the building supply is the owner's responsibility. You can use pvc or poly, but those are not allowed inside of the house. Pex is more expensive, and most people don't like that route.
Why are you worried about it?
P.S. there isn't really a standard location for PRV. Some put it at the meter, some directly after irrigation equipment, and some even put it at the side of the house or underneath.