Provide a more accurate count of max possible backtracks

[tjenkinson]

With ^(1|1|2|2)$ the tool will currently report 2 backtracks even though there can only be 1. This is because it doesn't check to see if the same input string could be used for both backtracks, which it can't.

^(1|1|2|2|[12])$ should result in 3.

Not sure how expensive this could be to calculate.