Italian car: the o-ring would be the problem, but you'd have to order it from Italy and some little old lady makes them by hand and for some odd reason, you'd have to disassemble significant parts of the interior to change it.
German car: the o-ring might be the only issue but they only sell it as an assembly that includes the pressure regulator, fuel pump, the left front headlight and some part they don't say what it is and you have no idea of where it goes or what it does.
American car: you can't replace the o-ring, only the regulator, but it's $8.00 at Pep Boys. You need a sledge hammer to R/R it, but otherwise, easy.