Named captures in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of named capture groups in Ruby regex

What is the output of this Ruby code using named captures?

Ruby

text = "Name: Alice, Age: 30"
pattern = /Name: (?<name>\w+), Age: (?<age>\d+)/
m = text.match(pattern)
puts "Name is #{m[:name]} and age is #{m[:age]}"

A Name is and age is 30

B Name is and age is

C Name is Alice and age is

D Name is Alice and age is 30

💡 Hint

Look at how named captures are accessed using the match object with symbols.

The regex uses named captures 'name' and 'age'. The match object allows access to these captures by their names as symbols, so m[:name] returns 'Alice' and m[:age] returns '30'.

❓ Predict Output

intermediate

Accessing named captures with MatchData#named_captures

What does this Ruby code print?

Ruby

text = "Product: Book, Price: 15"
pattern = /Product: (?<product>\w+), Price: (?<price>\d+)/
m = text.match(pattern)
p m.named_captures

A {"product"=>15, "price"=>"Book"}

B {:product=>"Book", :price=>"15"}

C {"product"=>"Book", "price"=>"15"}

D nil

💡 Hint

named_captures returns a hash with string keys and string values.

The named_captures method returns a hash where keys are strings of the capture names and values are the matched strings. So it returns {"product"=>"Book", "price"=>"15"}.

🔧 Debug

advanced

Identify the error in named capture usage

What error does this Ruby code raise?

Ruby

text = "User: John"
pattern = /User: (?<username>\w+)/
m = text.match(pattern)
puts m[:user]

A KeyError

B NoMethodError

C nil

D SyntaxError

💡 Hint

Check if the named capture key used matches the regex group name.

The named capture group is 'username', but the code tries to access m[:user]. This key does not exist, so Ruby raises a KeyError.

❓ Predict Output

advanced

Using named captures with multiple matches

What is the output of this Ruby code?

Ruby

text = "a1 b2 c3"
pattern = /(?<letter>\w)(?<digit>\d)/
text.scan(pattern) do |letter, digit|
  puts "Letter: #{letter}, Digit: #{digit}"
end

A

Letter: a, Digit: 1
Letter: b, Digit: 2
Letter: c, Digit: 3

B

Letter: a1, Digit: b2
Letter: c3, Digit: 

C SyntaxError

D

Letter: a, Digit: 1
Letter: b, Digit: 2

💡 Hint

scan returns all matches and yields captures as block parameters.

The pattern matches pairs of letter and digit. scan yields each pair to the block, printing each letter and digit on separate lines.

🧠 Conceptual

expert

Behavior of named captures with optional groups

Given this Ruby code, what is the value of m[:middle] after matching?

Ruby

text = "John Smith"
pattern = /(?<first>\w+) (?: (?<middle>\w+) )? (?<last>\w+)/x
m = text.match(pattern)
m[:middle].inspect

A "John"

B nil

C "Smith"

D " "

💡 Hint

Optional named capture groups return nil if not matched.

The middle name group is optional and not present in the text. So m[:middle] returns nil.