Gsub with regex in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the gsub method do in Ruby?

The gsub method replaces all occurrences of a pattern in a string with a specified replacement.

beginner

How do you use a regular expression with gsub?

You pass a regular expression as the first argument to gsub, and it replaces all matches of that pattern in the string.

beginner

Example: What is the result of "hello123".gsub(/\d/, "*")?

It replaces all digits with '*', so the result is "hello***".

intermediate

Can gsub use a block with regex? What does it do?

Yes, gsub can take a block. For each match, the block runs and returns the replacement string.

beginner

What is the difference between sub and gsub?

sub replaces only the first match, while gsub replaces all matches in the string.

What does "abc123".gsub(/\d/, "#") return?

A "abc123"

B "abc###"

C "###123"

D "abc#23"

All digits (\d) are replaced by '#', so '123' becomes '###'.

Which method replaces only the first match in a string?

A gsub

B replace

C sub

D match

sub replaces only the first occurrence, gsub replaces all.

What does the regex /\w+/ match?

A One or more word characters (letters, digits, underscore)

B Only digits

C Whitespace characters

D Special characters

\w+ matches one or more word characters like letters, digits, or underscore.

How can you use a block with gsub?

A To convert string to uppercase

B To ignore matches

C To delete the string

D To specify a dynamic replacement for each match

The block runs for each match and returns the replacement string dynamically.

What will "hello world".gsub(/o/, '0') output?

A "hell0 w0rld"

B "hello world"

C "hell0 world"

D "0ell0 w0rld"

All 'o' characters are replaced with '0'.

Explain how to use gsub with a regular expression to replace all vowels in a string with '*'.

Think about the regex pattern for vowels and how gsub applies replacements.

Describe the difference between using sub and gsub when working with regex in Ruby.

Focus on how many replacements each method performs.

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