Regular expressions with Regex class in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Regex object that matches the word "cat".

Kotlin

val pattern = Regex([1])
println(pattern.matches("cat"))

Drag options to blanks, or click blank then click option'

A"cat"

B"dog"

C"bat"

D"rat"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the pattern inside quotes.

Using a wrong pattern string.

The Regex constructor takes a string pattern. To match "cat", we pass "cat" as the pattern.

2fill in blank

medium

Complete the code to check if the string "hello123" contains digits using Regex.

Kotlin

val regex = Regex([1])
println(regex.containsMatchIn("hello123"))

Drag options to blanks, or click blank then click option'

A"[a-z]+"

B"\\d+"

C"[A-Z]+"

D"\\w+"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a pattern that matches only letters.

Not escaping the backslash properly.

The pattern "\\d+" matches one or more digits. It detects digits in the string.

3fill in blank

hard

Fix the error in the code to find all words starting with 'a' in the text.

Kotlin

val text = "apple banana apricot avocado"
val regex = Regex([1])
val matches = regex.findAll(text).map { it.value }.toList()
println(matches)

Drag options to blanks, or click blank then click option'

A"a+"

B"a*"

C"a\\w*"

D"a?"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "a*" which matches zero or more 'a's anywhere.

Using "a+" which requires at least one 'a' but not the following letters.

The pattern "a\\w*" matches words starting with 'a' followed by zero or more word characters.

4fill in blank

hard

Fill both blanks to create a Regex that matches email addresses and test it on a sample string.

Kotlin

val emailRegex = Regex([1])
val testEmail = "[e-mail]"
println(emailRegex.[2](testEmail))

Drag options to blanks, or click blank then click option'

A"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}"

Bmatches

CcontainsMatchIn

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using containsMatchIn() which checks partial matches.

Using an incomplete regex pattern.

The regex pattern matches common email formats. Using matches() checks if the whole string matches the pattern.

5fill in blank

hard

Fill all three blanks to extract all numbers from a string and convert them to integers.

Kotlin

val input = "I have 2 apples and 10 bananas"
val numberRegex = Regex([1])
val numbers = numberRegex.findAll(input).map { it.[2].to[3]() }.toList()
println(numbers)

Drag options to blanks, or click blank then click option'

A"\\d+"

Bvalue

CInt

Dtext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'text' instead of 'value' to get the matched string.

Forgetting to convert the string to Int.

The pattern "\\d+" finds sequences of digits. Each match's value is converted to Int using toInt().

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