Regular expressions with Regex class in Kotlin - Mini Project: Build & Apply

Regular expressions with Regex class

📖 Scenario: You are working on a simple text processing tool that needs to find specific patterns in user input. Regular expressions help you search for patterns like email addresses or phone numbers easily.

🎯 Goal: Build a Kotlin program that uses the Regex class to find all words starting with the letter 'a' in a given sentence.

📋 What You'll Learn

Create a string variable with a sentence containing multiple words

Create a Regex object to find words starting with 'a'

Use the findAll method of Regex to get all matches

Print all matched words separated by commas

💡 Why This Matters

🌍 Real World

Regular expressions are used in many apps to find patterns like emails, phone numbers, or keywords in text.

💼 Career

Knowing how to use Regex in Kotlin helps you build apps that process and validate text input efficiently.

1

Create the sentence string

Create a string variable called sentence and set it to the exact value: "An apple a day keeps the doctor away"

Kotlin

# Create a string variable called sentence with the given text
# Your code here

Hint

Use val sentence = "An apple a day keeps the doctor away" to create the string.

2

Create the Regex pattern

Create a Regex object called pattern with the exact pattern "\\ba\\w*" to find words starting with the letter 'a'.

Kotlin

val sentence = "An apple a day keeps the doctor away"
# Create a Regex object called pattern with pattern "\\ba\\w*"
# Your code here

Hint

Use val pattern = Regex("\\ba\\w*") to create the regex.

3

Find all matches using Regex

Use the findAll method of pattern on sentence and assign the result to a variable called matches.

Kotlin

val sentence = "An apple a day keeps the doctor away"
val pattern = Regex("\\ba\\w*")
# Use pattern.findAll(sentence) and assign to matches
# Your code here

Hint

Use val matches = pattern.findAll(sentence) to get all matches.

4

Print all matched words

Print all matched words separated by commas by mapping matches to their value and joining them with ", " inside a println.

Kotlin

val sentence = "An apple a day keeps the doctor away"
val pattern = Regex("\\ba\\w*")
val matches = pattern.findAll(sentence)
# Print matched words separated by commas
# Your code here

Hint

Use println(matches.map { it.value }.joinToString(", ")) to print the words.

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