How to Replace Text in a String in Kotlin Easily

In Kotlin, you can replace parts of a string using the replace() function. It lets you replace a substring or a character with another string or character easily. For example, val newString = oldString.replace("old", "new") replaces all occurrences of "old" with "new".

Syntax

The replace() function in Kotlin has two main forms:

• replace(oldValue: String, newValue: String, ignoreCase: Boolean = false): String - replaces all occurrences of oldValue with newValue.
• replace(oldChar: Char, newChar: Char, ignoreCase: Boolean = false): String - replaces all occurrences of a character.

The function returns a new string with replacements; the original string stays unchanged because strings are immutable.

kotlin

val original = "hello world"
val replaced = original.replace("world", "Kotlin")
println(replaced)

Output

hello Kotlin

Example

This example shows how to replace a word in a string and how to replace characters, including ignoring case sensitivity.

kotlin

fun main() {
    val text = "Hello World! Hello Kotlin!"

    // Replace substring
    val replacedText = text.replace("Hello", "Hi")
    println(replacedText)  // Replaces all "Hello" with "Hi"

    // Replace substring ignoring case
    val replacedIgnoreCase = text.replace("hello", "Hey", ignoreCase = true)
    println(replacedIgnoreCase)  // Replaces "Hello" ignoring case

    // Replace character
    val replacedChar = text.replace('!', '.')
    println(replacedChar)  // Replaces '!' with '.'
}

Output

Hi World! Hi Kotlin!
Hey World! Hey Kotlin!
Hello World. Hello Kotlin.

Common Pitfalls

One common mistake is expecting replace() to change the original string. Strings in Kotlin are immutable, so replace() returns a new string instead.

Another pitfall is forgetting to set ignoreCase = true when you want case-insensitive replacement.

kotlin

fun main() {
    val text = "Hello World"

    // Wrong: expecting original string to change
    text.replace("World", "Kotlin")
    println(text)  // Still prints "Hello World"

    // Right: assign the result
    val newText = text.replace("World", "Kotlin")
    println(newText)  // Prints "Hello Kotlin"

    // Case sensitive replacement fails
    val failReplace = text.replace("world", "Kotlin")
    println(failReplace)  // Prints "Hello World"

    // Correct with ignoreCase
    val successReplace = text.replace("world", "Kotlin", ignoreCase = true)
    println(successReplace)  // Prints "Hello Kotlin"
}

Output

Hello World
Hello Kotlin
Hello World
Hello Kotlin

Quick Reference

Function	Description	Example
replace(oldValue: String, newValue: String)	Replaces all occurrences of a substring	"text.replace(\"old\", \"new\")"
replace(oldValue: String, newValue: String, ignoreCase: Boolean)	Replaces substring ignoring case if true	"text.replace(\"old\", \"new\", ignoreCase = true)"
replace(oldChar: Char, newChar: Char)	Replaces all occurrences of a character	"text.replace('a', 'b')"

Key Takeaways

Use the replace() function to create a new string with replacements; original strings do not change.

You can replace substrings or characters with replace() in Kotlin.

Set ignoreCase = true to replace text without case sensitivity.

Always assign the result of replace() to a variable to keep the changed string.

replace() replaces all occurrences, not just the first one.