String.split() / joinToString()

// Splits a string by the given delimiter and returns a list.
string.split(delimiter, limit = maxSplits)

// Joins the elements of a collection with a separator and returns a string.
collection.joinToString(separator = separator, prefix = prefix, postfix = postfix)

fun main() {
    // Split by comma.
    val csv = "apple,orange,grape,peach"
    val fruits = csv.split(",")
    println(fruits) // [apple, orange, grape, peach]

    // Limit the number of splits.
    val limited = csv.split(",", limit = 2)
    println(limited) // [apple, orange,grape,peach]

    // Split by multiple delimiters.
    val text = "one two,three;four"
    val words = text.split(" ", ",", ";")
    println(words) // [one, two, three, four]

    // Join a list into a string.
    val list = listOf("Kotlin", "Java", "Swift")
    val joined = list.joinToString(separator = " / ")
    println(joined) // Kotlin / Java / Swift

    // Add a prefix and postfix.
    val withBrackets = list.joinToString(separator = ", ", prefix = "[", postfix = "]")
    println(withBrackets) // [Kotlin, Java, Swift]

    // Join with a transformation applied to each element.
    val numbers = listOf(1, 2, 3, 4, 5)
    val result = numbers.joinToString(separator = " + ", transform = { it.toString() })
    println(result) // 1 + 2 + 3 + 4 + 5
}

split() splits a string by a delimiter and returns a List<String>. To use a regular expression, pass a Regex object as the argument. If delimiters appear consecutively, empty strings will be included in the result list. To exclude them, use filter { it.isNotEmpty() }.

joinToString() joins the elements of a collection into a single string. All parameters have default values, so calling it without arguments joins elements with a comma separator. Use limit to cap the number of elements included; any excess is replaced with the value of truncated (default: "...").