String.substring() / drop() / take()

// By position
val sub: String = string.substring(startIndex)
val sub2: String = string.substring(startIndex, endIndex)  // endIndex is exclusive

// By character count from start or end
val d: String = string.drop(n)       // Returns the string with the first n characters removed
val t: String = string.take(n)       // Returns the first n characters
val dl: String = string.dropLast(n)  // Returns the string with the last n characters removed
val tl: String = string.takeLast(n)  // Returns the last n characters

fun main() {
    val s = "Hello, Kotlin!"

    // Extract a substring by specifying positions.
    println(s.substring(7))       // Kotlin!
    println(s.substring(7, 13))   // Kotlin (index 13 is not included)

    // Specify character counts with drop / take.
    println(s.take(5))            // Hello
    println(s.drop(7))            // Kotlin!
    println(s.takeLast(7))        // Kotlin!
    println(s.dropLast(7))        // Hello,

    // Specify a condition with dropWhile / takeWhile
    val csv = "000123"
    println(csv.dropWhile { it == '0' })  // 123 (leading zeros removed)
    println(csv.takeWhile { it == '0' })  // 000 (only leading zeros)

    // You can also get a substring using the range operator.
    println(s[0..4])              // Hello (indices 0 to 4)
}

Indices in substring() are zero-based, and the character at the end index is not included. Specifying an out-of-range index throws a StringIndexOutOfBoundsException.

drop() and take() are safe to use — they do not throw an exception when n is greater than the string length.

For checking string length, see string.length / isEmpty(). For splitting and joining, see string.split() / joinToString().