Kotlin extension function on String to find all instances of given substring within. Results are in the form of a list of index pairs.

StringFindAllInstances.kt

/**
 * Find all instances of given substring in this string.
 * Results are in the form of a list of index pairs (start and end index of that particular match).
 */
fun String.findAllInstancesOf(
    subString: String,
    startIndex: Int = 0
): List<Pair<Int, Int>> {
    val index = this.indexOf(subString, startIndex)
    if (index == -1) return emptyList()

    return listOf(index to index + subString.length)
        .plus(this.findAllInstancesOf(subString, index + 1))
}