Regex Matching in Kotlin

Introduction

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

Matches checks full string match. 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

• matches checks full string match.
• containsMatchIn checks partial match anywhere in text.
• matchEntire ensures complete string matches pattern.

Syntax

regex.matches(input)

Regex Matching in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val emailPattern = Regex("^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$")
    println(emailPattern.matches("user@example.com"))
    println(emailPattern.matches("invalid-email"))
}

Sample Output

true
false

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 emailPattern = Regex("^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$") assigns or updates a value used later in the program.

3. The println(emailPattern.matches("user@example.com")) statement writes a line to the console — this produces part of the sample output below.

4. The println(emailPattern.matches("invalid-email")) statement writes a line to the console — this produces part of the sample output below.

5. Matches checks full string match.

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: matches checks full string match.
• 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

• matches checks full string match.
• containsMatchIn checks partial match anywhere in text.
• matchEntire ensures complete string matches pattern.
• 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.