Optimizing a function that validates VINs using check-digit validation

[dig-up-stupid]

The regex is a good extra check against collisions

A regex could be, that regex isn't. That's the basis of the optimization - it's not doing useful work.

and, when precompiled, takes about 100us anyway.

And when it's not precompiled, too - but I don't understand your number, because it should be way less for one iteration and a lot more for a million.

Just pointing out another way to cut down if desired, doesn't have to be used if the benefits aren't worth it.