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matches and find

Learn regex search with matches and find.

matches and find: two ways to match

matches requires the whole string to fit the pattern; find searches inside the string and returns the first match.

val r = Regex("\\d+")
r.matches("123")   // true — all digits
r.matches("a12")   // false — not all digits
r.find("a12b")?.value  // "12" — first match

find returns null when nothing matches; that is why ?.value uses a safe call. Use matches to validate a phone, find to pull digits out of a longer text.

findAll: get every match

findAll returns every match in the string as a sequence. Each match is a MatchResult whose value holds the matched text.

val r = Regex("\\d+")
val list = r.findAll("a1b22c333").map { it.value }.toList()
// ["1", "22", "333"]

map turns each match into its value; toList collects them into a list. This is how you extract numbers, tags or codes from a text.

Practice Find and print the first digit

With Regex("\\d+"), call find on "a42b7"; print the value of the match.

Starter code

Main.kt
fun main() {
    val r = Regex("\\d+")
    // Call find for the first match and print its value
}

Sample solution

Main.kt
val r = Regex("\\d+")
println(r.find("a42b7")?.value)

Expected output

42

Questions and answers

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

1 What does Regex("\\d+").matches("abc") return?

  • null
  • true
  • false Correct answer
  • Throws an error

Explanation: matches returns false because the whole string is not digits; not null.

2 Fill in the blank: find first match

Question 
val r = Regex("\\d+")
val first = r.____("a1b2")
Answer 
val r = Regex("\\d+")
val first = r.find("a1b2")

Explanation: find returns the first match as a MatchResult.

3 Match method to job

Term Meaning
matches Whole must fit
find First match
findAll All matches
?.value Safe access

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