Regex literal syntax (/pattern/) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using regex literal?

Consider the following Ruby code snippet. What will it print?

Ruby

text = "Hello World"
pattern = /World/
puts text.match?(pattern)

A SyntaxError

B false

C nil

D true

💡 Hint

The regex literal /World/ matches the substring 'World' in the text.

The method match? returns true if the regex matches anywhere in the string. Since 'World' is present, it returns true.

❓ Predict Output

intermediate

What does this regex literal match in Ruby?

What will the following Ruby code output?

Ruby

text = "abc123xyz"
pattern = /\d{3}/
match = text.match(pattern)
puts match[0]

A 123

B \d{3}

C abc

D nil

💡 Hint

The regex /\d{3}/ looks for exactly three digits in a row.

The pattern matches '123' in the string, so match[0] returns '123'.

❓ Predict Output

advanced

What is the output of this Ruby regex literal with anchors?

What will this Ruby code print?

Ruby

text = "start middle end"
pattern = /^start.*end$/
puts text.match?(pattern)

A false

B true

C nil

D SyntaxError

💡 Hint

The regex uses ^ and $ anchors to match the whole string from start to end.

The pattern matches the entire string because it starts with 'start' and ends with 'end' with any characters in between.

❓ Predict Output

advanced

What error does this Ruby regex literal cause?

What error will this Ruby code raise?

Ruby

pattern = /unclosed
puts pattern

A SyntaxError

B No error, prints /unclosed/

C RuntimeError

D NameError

💡 Hint

Regex literals must be closed on the same line in Ruby.

The regex literal is not closed properly, causing a SyntaxError.

🧠 Conceptual

expert

How many matches does this Ruby regex literal find?

Given the code below, how many matches will be found?

Ruby

text = "one1 two2 three3 four4"
pattern = /\w+\d/
matches = text.scan(pattern)
puts matches.size

A 1

B 3

C 4

D 0

💡 Hint

The pattern matches a word followed by a digit.

The scan method finds all substrings matching the pattern. Here, 'one1', 'two2', 'three3', and 'four4' all match.

Practice

1. What is the correct way to write a regex literal in Ruby to match the word cat?

easy

A. regex('cat')

B. "cat"

C. 'cat'

D. /cat/

Solution

1. Step 1: Understand regex literal syntax

   In Ruby, regex literals are written between slashes, like /pattern/.

2. Step 2: Identify the correct pattern for 'cat'

   The pattern to match the word 'cat' is simply /cat/.

3. Final Answer:

   /cat/ -> Option D

4. Quick Check:

   Regex literal = /pattern/ [OK]

Hint: Regex literals always use slashes /pattern/ in Ruby [OK]

Common Mistakes:

• Using quotes instead of slashes
• Calling a non-existent regex() function
• Confusing string and regex syntax

2. Which of the following is the correct regex literal to match the word Dog case-insensitively in Ruby?

easy

A. /Dog/m

B. /Dog/g

C. /Dog/i

D. /Dog/

Solution

1. Step 1: Identify the case-insensitive option

   In Ruby regex, the i option after the slashes makes matching case-insensitive.

2. Step 2: Check the options given

   Only /Dog/i uses the correct i flag for case-insensitive matching.

3. Final Answer:

   /Dog/i -> Option C

4. Quick Check:

   Case-insensitive flag = i [OK]

Hint: Use /pattern/i for case-insensitive regex in Ruby [OK]

Common Mistakes:

• Using g or m flags which mean different things
• Omitting the i flag for case-insensitivity
• Placing flags inside the slashes

3. What will be the output of this Ruby code?

text = "Hello World"
match = text =~ /world/i
puts match

medium

A. nil

B. 6

C. 0

D. Error

Solution

1. Step 1: Understand the =~ operator with regex

   The operator =~ returns the index of the first match or nil if no match.

2. Step 2: Analyze the regex and string

   The regex /world/i matches 'World' ignoring case. In "Hello World", 'World' starts at index 6 (0-based).

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Case-insensitive match index = 6 [OK]

Hint: =~ returns match start index or nil if no match [OK]

Common Mistakes:

• Ignoring case-insensitive flag i
• Thinking output is true/false instead of index
• Assuming match starts at 0 always

4. Identify the error in this Ruby regex literal:

pattern = /hello/i/

medium

A. Extra slash at the end causes syntax error

B. Missing quotes around pattern

C. Missing i flag for case-insensitivity

D. Regex literals cannot have flags

Solution

1. Step 1: Check regex literal syntax

   A regex literal starts and ends with a single slash. Here, there is an extra slash at the end.

2. Step 2: Identify the syntax error

   The extra slash after /hello/i is invalid and causes a syntax error.

3. Final Answer:

   Extra slash at the end causes syntax error -> Option A

4. Quick Check:

   Regex literal must start and end with one slash pair [OK]

Hint: Regex literals have exactly two slashes, no extras [OK]

Common Mistakes:

• Adding extra slash after regex
• Confusing regex with string quotes
• Thinking flags go inside slashes

5. You want to find all words starting with a or A in a string using a regex literal. Which regex literal correctly matches this requirement?

hard

A. /\b[a]\w*/i

B. /\b[A]\w*/

C. /\b[a]\w*/

D. /\b[a-z]\w*/i

Solution

1. Step 1: Understand the requirement

   We want words starting with 'a' or 'A'. Using case-insensitive flag i simplifies matching both.

2. Step 2: Analyze options

   /\b[a]\w*/i uses /\b[a]\w*/i which matches word boundary, then 'a' (case-insensitive), then zero or more word chars. This matches words starting with 'a' or 'A'.

3. Final Answer:

   /\b[a]\w*/i -> Option A

4. Quick Check:

   Use i flag to match both 'a' and 'A' easily [OK]

Hint: Use /pattern/i to match letters ignoring case [OK]

Common Mistakes:

• Listing both 'a' and 'A' inside brackets without i flag
• Using wrong character classes
• Missing word boundary \b