String.replace() / trim()

// Replaces part of a string.
str.replace(oldValue: String, newValue: String, ignoreCase: Boolean = false)

// Removes whitespace from both ends of a string.
str.trim()
str.trimStart()  // Leading whitespace only
str.trimEnd()    // Trailing whitespace only

fun main() {
    // Replace a specific string.
    val text = "Hello, World! Hello, Kotlin!"
    val replaced = text.replace("Hello", "Hi")
    println(replaced) // Hi, World! Hi, Kotlin!

    // Replace while ignoring case.
    val noCase = text.replace("hello", "Hey", ignoreCase = true)
    println(noCase) // Hey, World! Hey, Kotlin!

    // Replace only the first occurrence.
    val firstOnly = text.replaceFirst("Hello", "Greetings")
    println(firstOnly) // Greetings, World! Hello, Kotlin!

    // Replace using a regular expression.
    val digits = "abc123def456"
    val noDigits = digits.replace(Regex("[0-9]+"), "#")
    println(noDigits) // abc#def#

    // Remove whitespace from both ends.
    val padded = "   Kotlin   "
    println(padded.trim())      // "Kotlin"
    println(padded.trimStart()) // "Kotlin   "
    println(padded.trimEnd())   // "   Kotlin"

    // Remove characters matching a condition.
    val slashed = "///path/to/file///"
    println(slashed.trim('/')) // path/to/file
}

『replace()』replaces all occurrences of a matching substring with a new string. By default, the match is case-sensitive, but you can pass 『ignoreCase = true』 to make it case-insensitive. To use a regular expression, pass a 『Regex』 object as the first argument.

『trim()』removes whitespace characters — spaces, tabs, newlines, and so on — by default. You can pass a lambda to specify a custom condition for which characters to remove. All of these methods return a new string; the original string is not modified. Strings in Kotlin are immutable.