Gsub with regex in Ruby - Mini Project: Build & Apply

Gsub with regex

📖 Scenario: You are working on a text editor feature that needs to clean up user input by replacing certain patterns in a string.

🎯 Goal: Learn how to use Ruby's gsub method with regular expressions to find and replace parts of a text.

📋 What You'll Learn

Create a string variable with a specific sentence.

Create a regular expression pattern to find all vowels.

Use gsub with the regex to replace vowels with a star *.

Print the modified string.

💡 Why This Matters

🌍 Real World

Text editors and input sanitizers often need to find and replace patterns in text, like removing vowels or masking sensitive data.

💼 Career

Understanding how to use regular expressions with string replacement is a valuable skill for software developers working on data cleaning, text processing, and user input validation.

1

Create the initial string

Create a string variable called sentence and set it to the exact value "Hello World! Welcome to Ruby programming."

Ruby

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

Hint

Use double quotes to create the string exactly as shown.

2

Create a regex pattern for vowels

Create a variable called vowels_regex and set it to a regular expression that matches all vowels (both uppercase and lowercase). Use /[aeiouAEIOU]/ as the pattern.

Ruby

sentence = "Hello World! Welcome to Ruby programming."
# Create the regex variable 'vowels_regex' to match vowels
# Your code here

Hint

Use slashes /.../ to create the regex pattern.

3

Replace vowels with stars using gsub

Use the gsub method on sentence with vowels_regex to replace all vowels with the star character '*'. Store the result in a new variable called cleaned_sentence.

Ruby

sentence = "Hello World! Welcome to Ruby programming."
vowels_regex = /[aeiouAEIOU]/
# Use gsub with vowels_regex to replace vowels with '*'
# Store the result in 'cleaned_sentence'
# Your code here

Hint

Call gsub on sentence with vowels_regex and '*' as arguments.

4

Print the cleaned sentence

Print the variable cleaned_sentence to display the sentence with vowels replaced by stars.

Ruby

sentence = "Hello World! Welcome to Ruby programming."
vowels_regex = /[aeiouAEIOU]/
cleaned_sentence = sentence.gsub(vowels_regex, '*')
# Print the cleaned_sentence
# Your code here

Hint

Use puts cleaned_sentence to print the result.

Practice

1. What does the Ruby method gsub do when used with a regular expression?

easy

A. It returns the length of the string.

B. It replaces all parts of the string that match the regex with a specified replacement.

C. It deletes the string entirely.

D. It splits the string into an array based on the regex.

Solution

1. Step 1: Understand gsub purpose

   The gsub method is used to replace parts of a string that match a pattern.

2. Step 2: Recognize regex usage

   When used with a regex, it finds all matches and replaces them with the given replacement string.

3. Final Answer:

   It replaces all parts of the string that match the regex with a specified replacement. -> Option B

4. Quick Check:

   gsub with regex = replace all matches [OK]

Hint: Remember: gsub replaces all regex matches in the string [OK]

Common Mistakes:

• Thinking gsub deletes the string
• Confusing gsub with split
• Assuming gsub returns string length

2. Which of the following is the correct syntax to replace all digits in a string with "#" using gsub and regex in Ruby?

easy

A. string.gsub("\d", "#")

B. string.gsub(/d/, "#")

C. string.gsub(/\d/, "#")

D. string.gsub("[0-9]", "#")

Solution

1. Step 1: Identify regex syntax in Ruby

   Regex patterns in Ruby are enclosed in slashes, like /pattern/.

2. Step 2: Match digits correctly

   The regex /\d/ matches any digit character. Using it with gsub replaces all digits.

3. Final Answer:

   string.gsub(/\d/, "#") -> Option C

4. Quick Check:

   Regex in slashes + \d for digits = string.gsub(/\d/, "#") [OK]

Hint: Use slashes / / for regex, not quotes " " [OK]

Common Mistakes:

• Using quotes instead of slashes for regex
• Using wrong regex like /d/ instead of /\d/
• Not escaping backslash properly

3. What is the output of this Ruby code?

text = "Call me at 123-456-7890."
result = text.gsub(/\d/, "*")
puts result

medium

A. Call me at ***-***-****.

B. Call me at 123-456-7890.

C. Call me at \d\d\d-\d\d\d-\d\d\d\d.

D. SyntaxError

Solution

1. Step 1: Understand the regex and replacement

   The regex /\d/ matches every digit character. The replacement is "*".

2. Step 2: Apply gsub to replace digits

   Every digit in the string is replaced by "*", so numbers become stars but other characters stay the same.

3. Final Answer:

   Call me at ***-***-****. -> Option A

4. Quick Check:

   Digits replaced by * = Call me at ***-***-****. [OK]

Hint: Digits \d replaced by * everywhere in string [OK]

Common Mistakes:

• Expecting only first digit replaced
• Confusing regex syntax causing error
• Thinking gsub changes non-digit characters

4. Identify the error in this Ruby code snippet:

sentence = "Hello World!"
sentence.gsub(/world/, "Earth")

What is the problem?

medium

A. Regex is case-sensitive, so 'world' does not match 'World'.

B. Missing replacement string in gsub.

C. Syntax error due to missing parentheses.

D. gsub cannot be used with regex.

Solution

1. Step 1: Check regex matching

   The regex /world/ is lowercase and Ruby regex is case-sensitive by default.

2. Step 2: Compare with string content

   The string has "World" with uppercase W, so the regex does not match anything.

3. Final Answer:

   Regex is case-sensitive, so 'world' does not match 'World'. -> Option A

4. Quick Check:

   Case sensitivity causes no match [OK]

Hint: Remember regex is case-sensitive unless flagged [OK]

Common Mistakes:

• Assuming regex matches ignore case by default
• Thinking gsub needs parentheses around arguments
• Believing gsub can't use regex

5. Given the string data = "User: John_Doe, Email: [e-mail]", which gsub call will replace all non-alphanumeric characters (except underscore) with a dash "-"?

hard

A. data.gsub(/[\w]/, "-")

B. data.gsub(/\w/, "-")

C. data.gsub(/[^a-zA-Z0-9]/, "-")

D. data.gsub(/[^a-zA-Z0-9_]/, "-")

Solution

1. Step 1: Understand character classes

   \w matches letters, digits, and underscore. \W matches anything not in \w.

2. Step 2: Analyze options

   data.gsub(/[^a-zA-Z0-9_]/, "-"): /[^a-zA-Z0-9_]/ matches non-alphanumeric except underscore - correct. data.gsub(/\w/, "-"): /\w/ replaces word chars (alnum + _) - wrong. data.gsub(/[^a-zA-Z0-9]/, "-"): /[^a-zA-Z0-9]/ replaces non-alnum including _ - wrong. data.gsub(/[\w]/, "-"): /[\w]/ replaces word chars - wrong.

3. Final Answer:

   data.gsub(/[^a-zA-Z0-9_]/, "-") -> Option D

4. Quick Check:

   Explicit negated class for alphanumeric + underscore = data.gsub(/[^a-zA-Z0-9_]/, "-") [OK]

Hint: Use negated class [^a-zA-Z0-9_] to keep underscore [OK]

Common Mistakes:

• Using /[^a-zA-Z0-9]/ which replaces underscores too
• Replacing word chars instead of non-word chars
• Confusing \w and \W meaning