// 1
var c = 0 // count of number of times this substitution is made in text
val newText = text.replace(re) {
    c += 1
    sub
}

// 2
val c = re.findAll(text).asSequence().count()
val newText = text.replace(re, sub)

val text = "abcdbdecbcd"
val matcher = "b".toPattern().matcher(text)
val sub = "Z"
var counter = 0
val newText = if(matcher.find()) {
	buildString(text.length) {
		do {
			counter++
			matcher.appendReplacement(this, sub)
		} while (matcher.find())
		matcher.appendTail(this)
	}
} else {
	text
}
// test:
check(counter == 3)
check(newText == "aZcdZdecZcd")

StringBuilder

re

c += 1

counter++

2

results()