ty for this example)

Because of this:

All of the ?s in an expression will represent the same digit (0-9), and it won't be one of the other given digits in the expression.

I wouldn't say you "shoudln't". It depends on how frequently this method is invoked. But this solution is way more readable than any of those performance optimized implementations.
Until performance becomes an issue, I favor readable/understandable implementations over fast ones.