How to use back references in kotlin regular expressions?

Question

I'm trying to use a regular expression with back references in kotlin to replace the placeholders of a String in the following fashion:

Source: "This is a %s with %02d whatever"

Target: "This is a <s/> with <02d/> whatever"

So I'm looking for something like this but with a proper syntax, of course:

private fun escapePlaceHolders(text: String): String {
    return """%([^ ]+?)""".toRegex().replace(text, "<\1/>")
}

Obviously this code doesn't even compile, let alone work. The problem is that I do not know how to use the back reference in the replace function, if it can be done at all.

Answer 1

The simplest way to do that is what Wiktor Stribi?ew described in the accepted answer.

There's a powerful alternative in case you need to not just reference but arbitrarily transform the matches for replacement, the replace overload with the signature:

fun CharSequence.replace(regex: Regex, transform: (MatchResult) -> CharSequence): String

It can be used as follows:

"""%([^ ]+)""".toRegex().replace(text) { "<${it.groupValues[1]}/>" }