Do I need a diode for reverse current protection using the ADP7142? I haven't found any information in the data sheet.
This particular regulator uses a P-channel MOSfet as the element that finally does the regulating in the control loop. While not shown, a MOSfet includes a body diode that provides the reverse protection function that you mention. Please note that this particular regulator protects itself - many other topologies do not.
You should be able to verfiy: an ohmmeter should be able to "diode-detect" between input pin and output pin (if it has a "diode" function).
The more common +ve voltage regulators often use a NPN bipolar transistor as the "pass element". These do need a protection diode, if the load voltage can possibly be larger than the source voltage. The NPN transistor's base-to-emitter diode can avalanche (with less than approx ten volts) and destroy or damage its function.
I am concerned with protecting the regulator, not with ** protecting the load** (from reverse current). Other methods are used to protect the load - many use a diode as well.
Yes some times you need diode it's depend you and you're design . if you don't want have change in output voltage and you're load generate the reverse current use free wheel diode like figure(1) and if you don't care about output voltage use figure(2).
decapod answers is good too.
Consider the situation that a large capacitor at the output maintains the output voltage after the input supply has been dropped. If this situation could damage the regulator then the manufacturer should have mentioned it. So a reverse current protection should not be needed