linear voltage regulator

My recommendation would be a diode in series with the +12V from the battery and a series resistor calculated to drop about 4 volts at the max current expected from the circuit. Using a 330uF filter capacitor (which I think was recommended earlier, the time constant should be long enough that the capacitor will not charge to above 35 volts during the duration of a short voltage spike. The capacitor should be rated for 35V (approx).

For additional protection, you could connect a 30V zener across the filter capacitor. The power rating of this will and the series resistor will depend on the magnitude of spikes you're expecting and their duration. As someone suggested, purpose designed transient supressors may be a better option, but they may be harder to source than a zener.

A disadvantage of this circuit may be the relatively slow rise time of the 5V rail. The solution to this depends on what you're circuit is doing and how it reacts to "brownout" conditions. Another disadvantage is the ability to work with a highly discharged battery (i.e. do you want it to continue to operate if the battery voltage falls to 10V?).

If those are significant issues, then I would replace the resistor with an NPN transistor having a Vce > 100V used as a pre-regulator. connect the collector to the cathode of the diode, the emitter to the 330uF capacitor. Connect a "suitable" resistor from collector to base and a zener diode (say, 8V) from base to ground. This transistor will regulate the voltage to the main regulator to a max of just under the zener voltage. The resistor from base to collector needs to supply sufficient base current to the transistor, but not so much that you waste significant power in the zener during large spikes.

Depending how well regulated you need the output voltage to be, you can use a 5.6V zener and dispense with the three terminal regulator. The component count will be similar, but the line and load regulation will be relatively poor.