[squarefoot]

You need to have an operator in the middle, that is, 12 would be satisfied by a string of "5+7", not just by adjacent 5 and 7.

[CaptainOfCoit]

What about if there is a `1` next to a `2`, shouldn't you be able to click `1` and then `2` and it concatenates to `12`?

I think that's what parent meant.

[squarefoot]

I didn't think of that, but wouldn't that be more about string operations than math?