A pressure regulator comprising: a valve body module including a valve body housing block retaining a poppet and a seat seal; a loading module including a loading block retaining a loading element configured to apply a load force to a sensing member disposed between the loading element and the poppet; a piston module including a piston adapter block secured to the valve body housing block and retaining the sensing member in engagement with the poppet, the pressure regulator being characterised in that the sensing member comprises a piston and the piston adapter block includes a piston bore retaining a stepped fluid driven portion of the piston; and the pressure regulator further comprises: a venting module including a venting adapter block secured between the piston adapter block and the loading block and defining a vent port in fluid communication with a vent passage in the sensing member; wherein the piston adapter block and the venting adapter block each include a uniformly sized first assembly interface, and the loading block and the venting adapter block each include a uniformly sized second assembly interface in mating and sealing engagement with the first assembly interface of the adjacent one of the piston adapter block and the venting adapter block, such that the piston adapter block is configured for assembly directly to the loading block after removing the venting module from between the piston adapter block and the loading block. a valve body module including a valve body housing block retaining a poppet and a seat seal; a loading module including a loading block retaining a loading element configured to apply a load force to a sensing member disposed between the loading element and the poppet; a piston module including a piston adapter block secured to the valve body housing block and retaining the sensing member in engagement with the poppet, the pressure regulator being characterised in that the sensing member comprises a piston and the piston adapter block includes a piston bore retaining a stepped fluid driven portion of the piston; and the pressure regulator further comprises: a venting module including a venting adapter block secured between the piston adapter block and the loading block and defining a vent port in fluid communication with a vent passage in the sensing member; wherein the piston adapter block and the venting adapter block each include a uniformly sized first assembly interface, and the loading block and the venting adapter block each include a uniformly sized second assembly interface in mating and sealing engagement with the first assembly interface of the adjacent one of the piston adapter block and the venting adapter block, such that the piston adapter block is configured for assembly directly to the loading block after removing the venting module from between the piston adapter block and the loading block. a venting module including a venting adapter block secured between the piston adapter block and the loading block and defining a vent port in fluid communication with a vent passage in the sensing member; wherein the piston adapter block and the venting adapter block each include a uniformly sized first assembly interface, and the loading block and the venting adapter block each include a uniformly sized second assembly interface in mating and sealing engagement with the first assembly interface of the adjacent one of the piston adapter block and the venting adapter block, such that the piston adapter block is configured for assembly directly to the loading block after removing the venting module from between the piston adapter block and the loading block.