Regular expressions with Regex class in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin Regex match?

Consider the following Kotlin code snippet. What will be printed when it runs?

Kotlin

val regex = Regex("\\d{3}-\\d{2}-\\d{4}")
val input = "My number is 123-45-6789"
println(regex.containsMatchIn(input))

A false

B true

C Compilation error

D Runtime exception

💡 Hint

The regex looks for a pattern of digits separated by dashes matching a social security number format.

The Regex pattern matches exactly three digits, a dash, two digits, a dash, and four digits. The input string contains "123-45-6789" which matches this pattern, so containsMatchIn returns true.

❓ Predict Output

intermediate

What does this Kotlin Regex replace produce?

What is the output of this Kotlin code?

Kotlin

val regex = Regex("cat|dog")
val text = "I have a cat and a dog."
val result = regex.replace(text, "pet")
println(result)

A I have a pet and a pet.

B I have a cat and a dog.

C I have a pet and a dog.

D I have a cat and a pet.

💡 Hint

The regex matches either "cat" or "dog" and replaces all matches.

The regex matches both "cat" and "dog" in the text. The replace function replaces all matches with "pet", so both words are replaced.

❓ Predict Output

advanced

What is the output of this Kotlin Regex findAll usage?

What will this Kotlin code print?

Kotlin

val regex = Regex("\\b\w{4}\b")
val text = "This test code finds four letter words."
val matches = regex.findAll(text).map { it.value }.toList()
println(matches)

A [This, test, code, find, four, word]

B [This, test, code, find, four, word, words]

C [This, test, code, four]

D [This, test, code]

💡 Hint

The regex looks for exactly 4-letter words using word boundaries.

The regex matches words with exactly 4 letters. "This", "test", "code", and "four" are 4-letter words. "find" has 4 letters but is followed by 's' in "finds", so it doesn't match as a whole word. "word" is 4 letters but appears as "words" (5 letters).

❓ Predict Output

advanced

What error does this Kotlin Regex code raise?

What error will this Kotlin code produce when run?

Kotlin

val regex = Regex("[A-Z+")
val text = "Hello"
println(regex.matches(text))

A java.util.regex.PatternSyntaxException

B No error, prints false

C No error, prints true

D NullPointerException

💡 Hint

Check if the regex pattern is valid.

The regex pattern "[A-Z+" is missing a closing bracket for the character class, causing a PatternSyntaxException at runtime.

🧠 Conceptual

expert

How many matches does this Kotlin Regex find?

Given the Kotlin code below, how many matches will be found?

Kotlin

val regex = Regex("(?<=\\s)\d+(?=\\s)")
val text = " 123 4567 89 "
val matches = regex.findAll(text).count()
println(matches)

A 0

B 1

C 3

D 2

💡 Hint

Look carefully at the lookbehind and lookahead requiring spaces around digits.

The regex matches digits that have a space before and after. "123" and "4567" are surrounded by spaces, so they match. "89" is followed by a space but preceded by '7' (no space), so it does not match. Hence, 2 matches.