Regex Replace in Kotlin

Introduction

Regex Replace is a fundamental concept every Kotlin developer should understand. Regular expressions match, search, and replace text patterns — useful for validation, parsing, and log processing.

Replace function substitutes matched text with replacement string. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• replace function substitutes matched text with replacement string.
• replaceFirst replaces only first match.
• Useful for cleanup and formatting.

Syntax

text.replace(regex, "new")

Regex Replace in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val text = "Phone: 123-456-7890"
    val cleaned = text.replace(Regex("[^0-9]"), "")
    println(cleaned)
}

Sample Output

1234567890

When to use

Use regex when simple string methods are not enough — email validation, log parsing, or token extraction.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val text = "Phone: 123-456-7890" assigns or updates a value used later in the program.

3. val cleaned = text.replace(Regex("[^0-9]"), "") assigns or updates a value used later in the program.

4. The println(cleaned) statement writes a line to the console — this produces part of the sample output below.

5. Replace function substitutes matched text with replacement string.

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: replace function substitutes matched text with replacement string.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• replace function substitutes matched text with replacement string.
• replaceFirst replaces only first match.
• Useful for cleanup and formatting.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.