I'm trying to get a string like:
"1d6+1d12-1d20+5"
And turn it into this: "rolar('1d6')+rolar('1d12')-rolar('1d20')+5"
The gist being that the xdy should be replaced with rolar('xdy') but with the original value in that part. I've been trying to do this with string:gsub but haven't been able to mantain the original xdy value.
The pattern for this is simply "one or more digits, then the letter d, then again one or more digits". Replace this with the capture wrapped in a string literal & func call to rolar.
str:gsub("%d+d%d+", "rolar('%1')")
where str is your string - for example:
> ("1d6+1d12-1d20+5"):gsub("%d+d%d+", "rolar('%1')")
rolar('1d6')+rolar('1d12')-rolar('1d20')+5 3
(the second return value, 3, is the number of replacements performed by gsub; you presumably don't need it)
I assume your next step will be to loadstring this code in order to evaluate it. Be very careful when doing so; at the very least use a sandboxed environment (i.e., an environment without access to any global variables, ideally even with a temporarily changed string metatable). This won't protect against even simple DoS attacks like while 1 do end, but at least it doesn't make it trivial to wipe your filesystem.
If you're positive that the input will always be in a configuration like that, you can do this with gsub, using capture groups (parentheses around string patterns).
str:gsub("(%d+d%d+)", function(roll) return "rolar('"..roll.."')" end)
The captured pattern %d+d%d is passed to the function you give to gsub as an argument, and you can then re-insert back into the string with whatever modifications you want.