Reroll Exactly One Die, Once

One of the characters in my campaign has an ability to reroll a damage die which results in 1, but only one die, and the reroll value is kept. This is different from, say, "brutal" weapons which always reroll 1s. As far as I can tell, there's no way to do that. Is there any way to drop a single 1 from a roll and trigger a reroll?

I know there is a feat for it. That allows characters to reroll a 1 once a day.

Not even once per day. That at least would come up once in a while. The feat in question is Axe Expertise, which in addition to the +1 bonus per tier also reads " Also, when rolling damage for a weapon attack you make with an axe, you can reroll one damage die that results in a 1, but you must use the second result." Since there's basically no reason NOT to use it, it would be nice to automate it. The only issue would be a consistent syntax. Right now I could do, say, 6d10r1 -- roll 6d10 and reroll the 1s. This handles brutal weapons just fine but isn't what I want. Then you might do something like 6d10r1n1. This "n" modifies the "r" to tell you that you're getting exactly one "reoll one" This could still be ambiguous though. What happens if you have 6d10r<2n1. I would re-read that as reroll any one die which is less than 2, but you roll a 1 and a 2, you then need to decide which one of those you roll. Ideally it's the lowest, right? That seems sane default behavior. But should you be changing that? What if the reroll condition is to reroll dice OVER a threshold instead of under? So... yeah. Not easy. Thoughts welcome.

I implemented this when we were playing 4e via rpTools by having it record each of the individual rolls for the (w) into an array, checking the array to see if it contains at least one "1", then if so, subtract 1 from the running total and add another (w) to the damage total. It spit out a chat line too so the player would get feedback on their feat helping them out. Since you're a mentor, you could use an API script that does this, I don't have the ability to provide one for you though. i.e. roll 3(w) +4 (str) +2 (enh) w/ axe expertise, in this case (w)=12 roll an array of {3,8,1} check to see if the array contains at least one 1, which it does remove a single 1, roll another 1d12 and add to the array spit out final result of 12 (w) + 4 (str) +2 (enh) = 18 If memory serves, this would also apply to the bonus dice from crit damage, so perfect implementation would actually need to collect all dice rolled as part of the damage, track their dice type, etc etc etc, which was too much work for me. I figured having it work on non-crits was good enough, crits missing a teensy bit of damage wasn't making the difference.