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Regex

Learn regular expressions with Regex.

Regex: matching with patterns

In Kotlin the Regex class matches text against a pattern. A pattern is a string with special symbols: \d digit, \w letter digit underscore, + one or more, * zero or more.

val digits = Regex("\\d+")
val result = digits.matches("12345")
// true

Inside a normal string you must double the backslash. With a raw string a single backslash is enough. Regex is ideal for validating phone, email or date-like strings.

Using the Regex class

A Regex object is created with Regex(pattern). If you use the same pattern many times, store it in a variable so it is not recompiled.

val phone = Regex("\\d{3}-\\d{4}")
println(phone.matches("555-1234"))
// true

In a pattern {n} means exactly n times, {n,m} means at least n at most m. This lets you write fixed-length patterns easily. Define the pattern once, then match as many times as you need.

Practice Test with a Regex

Define a Regex that matches one or more digits. Test the string "2024" with matches and print the result.

Starter code

Main.kt
fun main() {
    // Define a Regex: one or more digits
    // Test the string "2024" with matches
}

Sample solution

Main.kt
val r = Regex("\\d+")
println(r.matches("2024"))

Expected output

true

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match pattern to meaning

Term Meaning
\d Digit
\w Letter digit underscore
+ One or more
* Zero or more

2 Fill in the blank: define a Regex

Question 
val r = ____("\\d+")
Answer 
val r = Regex("\\d+")

Explanation: Regex(pattern) creates a new pattern object.

3 Which pattern captures the digits in 'abc123'?

Main.kt
val r = Regex("\\d+")
  • \d?
  • \w-
  • [abc]+
  • \d+ Correct answer

Explanation: \d+ matches one or more digits.

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together