Regular expressions with Regex class in Kotlin

Introduction

Regular expressions help you find patterns in text quickly. The Regex class in Kotlin makes it easy to search, match, and replace text using these patterns.

You want to check if a phone number is in the right format.

You need to find all email addresses in a block of text.

You want to replace all spaces with dashes in a string.

You want to split a sentence into words based on punctuation.

You want to validate if a password contains certain characters.

Syntax

Kotlin

val regex = Regex("pattern")

// Check if text matches pattern
val result = regex.matches("text")

// Find first match
val match = regex.find("text")

// Find all matches
val matches = regex.findAll("text")

// Replace matches
val replaced = regex.replace("text", "replacement")

The pattern is a string that describes what you want to find.

Use double backslashes \\ in the pattern to escape special characters.

Examples

This finds the first group of digits in the text and prints it.

Kotlin

val regex = Regex("\\d+")
val text = "There are 123 apples"
println(regex.find(text)?.value)

This finds all words made of letters and prints them as a list.

Kotlin

val regex = Regex("[a-zA-Z]+")
val text = "Hello 123 World"
val words = regex.findAll(text).map { it.value }.toList()
println(words)

This replaces all occurrences of "cat" with "dog" in the text.

Kotlin

val regex = Regex("cat")
val text = "The cat sat on the cat mat"
val replaced = regex.replace(text, "dog")
println(replaced)

Sample Program

This program finds all words with exactly 4 letters in the given text and prints them as a list.

Kotlin

fun main() {
    val regex = Regex("\\b\\w{4}\\b") // words with exactly 4 letters
    val text = "This test will find four letter words like test and find"
    val matches = regex.findAll(text).map { it.value }.toList()
    println("Four letter words found: $matches")
}

Important Notes

Regex patterns can be tricky at first; try simple patterns and test often.

Use online regex testers to experiment with patterns before coding.

The Regex class methods like findAll return sequences, so convert to list to see all matches.

Summary

Regex class helps find and work with text patterns easily.

Use matches, find, findAll, and replace methods for common tasks.

Patterns use special symbols; escape them properly in Kotlin strings.