Named captures in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a named capture in Ruby regular expressions?

A named capture lets you give a name to a part of a regular expression match, so you can access it by that name instead of by number.

beginner

How do you define a named capture group in a Ruby regex?

Use the syntax (?<name>pattern) where name is the capture's name and pattern is what you want to match.

beginner

How do you access the value of a named capture after a match in Ruby?

You can use MatchData#[] with the capture name as a symbol or string, like match[:name] or match["name"].

beginner

What does this Ruby code output?

text = "Name: Alice"
if text =~ /Name: (?<user>\w+)/
  puts "User is #{Regexp.last_match[:user]}"
end

It outputs: User is Alice because the named capture user matches "Alice" and is accessed with Regexp.last_match[:user].

beginner

Why are named captures useful compared to numbered captures?

Named captures make code easier to read and maintain because you use meaningful names instead of numbers, which can be confusing especially with many groups.

Which syntax defines a named capture group in Ruby regex?

A (name:pattern)

B (?<name>pattern)

C (name=pattern)

D (pattern?<name>)

The correct syntax for named captures in Ruby is (?pattern).

How do you access a named capture called 'id' from a MatchData object named 'match'?

A match.id

B match.get('id')

C match['id']()

D match[:id]

You access named captures using square brackets with the name as a symbol or string, like match[:id].

What will this code print?

text = "Age: 30"
if text =~ /Age: (?<age>\d+)/
  puts Regexp.last_match[:age]
end

A age

B Age

C 30

D Error

The named capture 'age' matches '30', so it prints '30'.

Why might you prefer named captures over numbered captures?

A They make code easier to understand by using meaningful names.

B They run faster than numbered captures.

C They allow matching multiple patterns at once.

D They automatically convert strings to numbers.

Named captures improve readability by using descriptive names instead of numbers.

Which of the following provides access to the MatchData object after a successful regex match in Ruby?

A All of the above

B Regexp.last_match

C String#match

D =~ operator

All these can provide MatchData: =~ operator sets Regexp.last_match, and String#match returns MatchData.

Explain how to create and use named captures in Ruby regular expressions.

Think about the <code>(?<name>pattern)</code> syntax and accessing with MatchData.

Describe the benefits of using named captures instead of numbered captures in Ruby regex.

Consider how meaningful names help when you have many groups.

Practice

1. What does a named capture group in Ruby regex do?
/(?<name>pattern)/

easy

A. Repeats the pattern multiple times

B. Makes the regex case-insensitive

C. Labels a part of the matched text with a name for easy access

D. Matches only digits in the text

Solution

1. Step 1: Understand named capture syntax

   The syntax ?<name> inside parentheses names the capture group.

2. Step 2: Purpose of named captures

   This lets you access matched parts by name instead of number, making code clearer.

3. Final Answer:

   Labels a part of the matched text with a name for easy access -> Option C

4. Quick Check:

   Named capture = label matched text [OK]

Hint: Named captures label groups for easy access by name [OK]

Common Mistakes:

• Confusing named captures with regex flags
• Thinking named captures repeat patterns
• Assuming named captures only match digits

2. Which of the following is the correct syntax for a named capture group in Ruby regex?

easy

A. /<year>\d{4}/

B. /(?year>\d{4})/

C. /(?/

D. /(?<year>\d{4})/

Solution

1. Step 1: Recall named capture syntax

   Named captures use ?<name> inside parentheses to name the group.

2. Step 2: Check each option

   /(?<year>\d{4})/ correctly uses (?<year>\d{4}). Others miss brackets or have wrong syntax.

3. Final Answer:

   /(?<year>\d{4})/ is correct syntax -> Option D

4. Quick Check:

   Correct syntax = (?<name>) [OK]

Hint: Look for parentheses with ?<name> inside [OK]

Common Mistakes:

• Omitting angle brackets around the name
• Placing name outside parentheses
• Using wrong brackets or missing parentheses

3. What is the output of this Ruby code?

text = "Date: 2024-06-15"
regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
m = text.match(regex)
puts "Year: #{m[:year]}, Month: #{m[:month]}, Day: #{m[:day]}"

medium

A. Year: 2024, Month: 06, Day: 15

B. Year: , Month: , Day:

C. Year: 2024-06-15, Month: 2024-06-15, Day: 2024-06-15

D. Error: undefined method [] for nil

Solution

1. Step 1: Match regex with named captures

   The regex captures year, month, and day parts from the text "Date: 2024-06-15".

2. Step 2: Access named captures from match object

   Using m[:year], m[:month], and m[:day] returns "2024", "06", and "15" respectively.

3. Final Answer:

   Year: 2024, Month: 06, Day: 15 -> Option A

4. Quick Check:

   Named captures return parts by name [OK]

Hint: Use m[:name] to get named capture values [OK]

Common Mistakes:

• Using m['name'] instead of m[:name]
• Forgetting to call match before accessing captures
• Expecting full match instead of named parts

4. Find the error in this Ruby code using named captures:

text = "User: alice, Age: 30"
regex = /User: (?<name>\w+), Age: (?<age>\d+)/
m = regex.match(text)
puts "Name: #{m['name']}, Age: #{m['age']}"

medium

A. Regex pattern missing parentheses

B. Using string keys instead of symbols to access named captures

C. Match method called on regex instead of string

D. No error, code runs fine

Solution

1. Step 1: Check how named captures are accessed

   Named captures in Ruby match objects are accessed with symbols, like m[:name], not strings.

2. Step 2: Identify the error in code

   The code uses m['name'] and m['age'], which causes a nil result or error.

3. Final Answer:

   Using string keys instead of symbols to access named captures -> Option B

4. Quick Check:

   Use symbols, not strings, for named captures [OK]

Hint: Access named captures with symbols like m[:name] [OK]

Common Mistakes:

• Using string keys instead of symbols
• Confusing match method call on regex vs string
• Missing parentheses in regex groups

5. Given this text:

"Order #1234: 5 items, total $99.95"

Write a Ruby regex with named captures to extract order number, item count, and total price.
Which regex correctly captures these parts?

hard

A. /Order #(?<order_num>\d+): (?<items>\d+) items, total \$(?<total>\d+\.\d{2})/

B. /Order #(?<order_num>\d+), (?<items>\d+) items total \$(?<total>\d+\.\d{2})/

C. /Order #(?<order_num>\d+): (?<items>\d+) items total \$(?<total>\d+\.\d{2})/

D. /Order #(?<order_num>\d+): (?<items>\d+) items, total \$(?<total>\d+\.\d{3})/

Solution

1. Step 1: Analyze the text format

   The text is "Order #1234: 5 items, total $99.95" with colon and comma exactly as shown.

2. Step 2: Match regex parts carefully

   /Order #(?<order_num>\d+): (?<items>\d+) items, total \$(?<total>\d+\.\d{2})/ matches colon after order number, space, number of items, comma, space, "total $", and price with two decimals.

3. Step 3: Check other options

   Options A and C miss the comma or colon or both, and /Order #(?<order_num>\d+): (?<items>\d+) items, total \$(?<total>\d+\.\d{3})/ expects three decimals instead of two.

4. Final Answer:

   /Order #(?<order_num>\d+): (?<items>\d+) items, total \$(?<total>\d+\.\d{2})/ -> Option A

5. Quick Check:

   Match exact punctuation and decimal places [OK]

Hint: Match exact punctuation and decimal digits in regex [OK]

Common Mistakes:

• Missing colon or comma in pattern
• Wrong decimal places in price capture
• Using incorrect group names or syntax