I think that the (??{...}) would be best choice as repositioning the pos() is probably not a good idea in general.

What's the problem with setting pos()? (??{...}) and (?{...}) have their share of problems (many related to scoping of variables), and I avoid them as much as possible - and if I use them, I keep the code it in as simple as possible.

What I don't understand is your use of 'last'. It means that if you have "+2A." in your string, nothing following it will change.