Steve, the regulator could be bad. But test the aircraft wiring before replacing the regulator. Do the tests described HERE.
Then use a high wattage 12 volt lamp like a brake light or lawnmower headlight and connect one lamp wire to the regulator case (not to aircraft ground nor to battery negative). And connect the other lamp wire to regulator terminals B and R and C, (one terminal at a time). The lamp should illuminate brightly. Also measure the voltage across the lamp while it is on. The voltage should equal battery voltage. Again, use the regulator case as a voltage reference, not aircraft ground. If no problem is found with aircraft wiring, then replace the regulator. Loading a circuit with a lamp will make problems show up that might not show up with just a voltmeter alone.
Other possible causes of low voltage could be an open alternator circuit, or a blown main fuse, or a half-bad master switch. If you replace the regulator, use heat-conductive grease between the case and the airframe.