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.

Practice

1. What does the Regex.matches() function do in Kotlin?

easy

A. Finds the first occurrence of the pattern in the string

B. Splits the string by the regex pattern

C. Replaces all matches of the pattern with a new string

D. Checks if the entire input string matches the regex pattern

Solution

1. Step 1: Understand Regex.matches() purpose

   The matches() function returns true only if the whole input string fits the pattern exactly.

2. Step 2: Differentiate from other Regex methods

   Other methods like find() look for parts of the string, but matches() requires full match.

3. Final Answer:

   Checks if the entire input string matches the regex pattern -> Option D

4. Quick Check:

   matches() = full string match [OK]

Hint: matches() means full string must fit pattern exactly [OK]

Common Mistakes:

• Thinking matches() finds partial matches
• Confusing matches() with find()
• Assuming matches() replaces text

2. Which of the following is the correct way to create a Regex object to match digits in Kotlin?

easy

A. val regex = Regex("\\d+")

B. val regex = Regex("\d+")

C. val regex = Regex("\d")

D. val regex = Regex("[d]+")

Solution

1. Step 1: Understand escaping in Kotlin strings

   In Kotlin, backslash must be escaped, so to represent \d (digit), we write "\\d".

2. Step 2: Check the quantifier usage

   "\\d+" means one or more digits, which is correct for matching digits.

3. Final Answer:

   val regex = Regex("\\d+") -> Option A

4. Quick Check:

   Double backslash needed for \d in Kotlin strings [OK]

Hint: Escape backslash twice in Kotlin regex strings [OK]

Common Mistakes:

• Using single backslash without escaping
• Confusing character class syntax
• Missing quantifier '+' for multiple digits

3. What will be the output of this Kotlin code?

val regex = Regex("a.b")
val input = "acb aab aeb"
val matches = regex.findAll(input).map { it.value }.toList()
println(matches)

medium

A. ["acb", "aab", "aeb"]

B. ["acb", "aab"]

C. ["aab", "aeb"]

D. []

Solution

1. Step 1: Understand the regex pattern "a.b"

   The dot '.' matches any single character, so "a.b" matches 'a' + any char + 'b'.

2. Step 2: Check input string matches

   Input has "acb", "aab", and "aeb" all matching the pattern.

3. Final Answer:

   ["acb", "aab", "aeb"] -> Option A

4. Quick Check:

   Dot matches any char between 'a' and 'b' [OK]

Hint: Dot '.' matches any single character in regex [OK]

Common Mistakes:

• Thinking dot matches zero or multiple chars
• Missing some matches in the list
• Confusing findAll with matches

4. Identify the error in this Kotlin code snippet:

val regex = Regex("[a-z")
val result = regex.matches("abc")

medium

A. Missing escape for square brackets

B. Unclosed character class in regex pattern

C. matches() cannot be used with character classes

D. matches() requires a raw string literal

Solution

1. Step 1: Check regex pattern syntax

   The pattern "[a-z" has an opening square bracket but no closing bracket, causing a syntax error.

2. Step 2: Understand consequences

   Regex constructor will throw an exception due to invalid pattern.

3. Final Answer:

   Unclosed character class in regex pattern -> Option B

4. Quick Check:

   Character classes must be closed with ']' [OK]

Hint: Always close character classes with ']' in regex [OK]

Common Mistakes:

• Forgetting to close square brackets
• Thinking matches() needs raw strings
• Confusing escape rules for brackets

5. You want to replace all sequences of one or more spaces in a string with a single dash ('-'). Which Kotlin Regex and method combination correctly does this?

hard

A. Regex("\\s+").replaceAll(input, "-")

B. Regex("\\s*").replace(input, "-")

C. Regex("\\s+").replace(input, "-")

D. Regex("[ ]+").replaceAll(input, "-")

Solution

1. Step 1: Choose correct regex pattern for spaces

   "\\s+" matches one or more whitespace characters; correct for sequences of spaces.

2. Step 2: Use correct Kotlin Regex method

   Kotlin's Regex class uses replace() to replace all matches; replaceAll() does not exist.

3. Final Answer:

   Regex("\\s+").replace(input, "-") -> Option C

4. Quick Check:

   Use replace() with "\\s+" to replace spaces [OK]

Hint: Use replace() with "\\s+" to replace space sequences [OK]

Common Mistakes:

• Using replaceAll() which is not in Kotlin Regex
• Using "\\s*" which matches zero spaces
• Not escaping backslash properly