An isolation sub for use in a perforating system that includes a pressure activated firing head. The isolation sub is set between the pressure source that is used to initiate the firing head. A pressure regulator in the sub is responsive to fluctuations in pressure difference between the pressure source and wellbore and isolates the firing head when the pressure difference is at or approaches a designated pressure difference that could initiate the firing head. The pressure regulator includes a spring loaded piston that seals the firing head from the source pressure before the pressure difference activates the firing head.