by adjusting the range

You could use

StringBuilder

instead.

val sb = StringBuilder()
sb.add(orig.substring(0, rng0.start)
sb.add(repl[0])
sb.add(orig.substring(rng0.end + 1, rng1.start)
// etc

Just check my ranges, I might have a few off by one errors in there. I don’t remember when start/end are inclusive or exclusive. Also the ranges can’t overlap but you can just modify them

ah yes ty