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.

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