There are any number of ways to control 12V by 5V.

For example use the PIC to drive an npn TR to ground the base of a
pnp power TR whose base is normally held high by a  resistor (the
pnp's emitter is also connected directly to 12V), the 12V coming out
of the pnp collector into the reg.

Or an optocoupler, driving the LED side with the PIC, the TR side
driving either a power TR, FET or reg directly

Or a 5V-12V level shifter, eg ICL7667 to drive a power FET

Or even an RS232-type or bus level shifter (you may have one lying
around)

etc etc etc

A lot depends on what current you'll be expecting to put through the reg

Jinx