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Kotlin String Operations

String operations cut a string into pieces and pull chosen pieces out of it. Each operation returns a new value and leaves the original string unchanged.

Splitting a string into pieces

The call split() cuts the string at each comma. The result is a list, so its size and its parts can be read like any list.

fun main() {
    val line = "apples,pears,plums,figs"
    val parts = line.split(",")
    println(parts)
    println(parts.size)
    println(parts[1])
}

Splitting with a limit

A limit of 3 stops the cutting after two cuts. The third piece keeps the rest of the string whole.

fun main() {
    val text = "one-two-three-four"
    val parts = text.split("-", limit = 3)
    println(parts)
    println(parts[2])
}

Taking text before and after a mark

The call substringBefore() keeps the text before the first mark. The call substringAfter() keeps the text after it.

fun main() {
    val email = "ada@example.com"
    println(email.substringBefore("@"))
    println(email.substringAfter("@"))
    println("readme".substringAfter("."))
}

Copying a range and trimming the ends

The call substring() with a range copies both end characters of the range. The calls removePrefix() and removeSuffix() drop a piece only when it sits at that end.

fun main() {
    val code = "KT-4821-INT"
    println(code.substring(3..6))
    val file = "temp-report.txt"
    println(file.removePrefix("temp-"))
    println(file.removePrefix("temp-").removeSuffix(".txt"))
}

A split and join round trip

Splitting a string and joining the pieces rebuilds a string. A different separator gives the same words in a new shape.

fun main() {
    val words = "the quick brown fox"
    val parts = words.split(" ")
    println(parts)
    println(parts.joinToString(" "))
    println(parts.joinToString("-"))
}

Example

fun main() {
    val address = "Ada Lovelace, 12 Analytical Lane, London"
    val parts = address.split(", ")
    println(parts)
    println(address.substringBefore(","))
    println(address.substringAfter(", "))
    println(parts.joinToString(" / "))
}

Key points

  • Use split() to cut a string at a delimiter and get a list of the pieces.
  • Give split() a limit to cap the number of pieces, and the last piece keeps the rest of the string.
  • Use substringAfter() to take the text after the first match of a delimiter.
  • Use substringBefore() to take the text before the first match of a delimiter.
  • Use substring() with a range to copy the characters inside that range.
  • Use removePrefix() and removeSuffix() to drop a known piece from one end of a string.

In this example one address line is cut in several ways. Split gives the three parts as a list, substringBefore() isolates the name, substringAfter() keeps everything past the first comma, and joinToString() rebuilds the line with a new separator.

Note

When a delimiter is missing, substringAfter() and substringBefore() return the whole string.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

A program splits "red,green,blue" on "," with a limit of 2. What does printing the resulting list show?