Named captures in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a named capture group called 'year' in the regex.

Ruby

regex = /(?<[1]>\d{4})/  # Matches a 4-digit year

Drag options to blanks, or click blank then click option'

Adate

Byear

Cmonth

Dday

Attempts:

3 left

💡 Hint

Common Mistakes

Using unnamed capture groups without the ? syntax.

Using incorrect group names like 'date' or 'month' when the pattern matches a year.

The syntax (?pattern) defines a named capture group called 'year' that matches 4 digits.

2fill in blank

medium

Complete the code to access the named capture 'month' from the match data.

Ruby

match = /(?<month>\d{2})/.match('07')
month = match[:[1]]

Drag options to blanks, or click blank then click option'

Amonth

Bdate

Cyear

Dday

Attempts:

3 left

💡 Hint

Common Mistakes

Using string keys instead of symbols.

Using the wrong capture group name.

You access named captures from a MatchData object using symbols like match[:month].

3fill in blank

hard

Fix the error in the code to correctly define and use a named capture 'day'.

Ruby

regex = /(?<[1]>\d{2})/
match = regex.match('15')
day = match[:day]

Drag options to blanks, or click blank then click option'

Amonth

Bdate

Cday

Dyear

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatch between capture group name and symbol used to access it.

Using an undefined capture group name.

The named capture group must be called 'day' to match the symbol used to access it.

4fill in blank

hard

Fill both blanks to create a regex with named captures 'year' and 'month' and access them.

Ruby

regex = /(?<[1]>\d{4})-(?<[2]>\d{2})/
match = regex.match('2023-06')
year = match[:year]
month = match[:month]

Drag options to blanks, or click blank then click option'

Ayear

Bday

Cmonth

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong capture group names that don't match the symbols used later.

Mixing up the order of year and month.

The regex defines named captures 'year' and 'month' to match the parts of the date string.

5fill in blank

hard

Fill all three blanks to create a regex with named captures 'year', 'month', 'day' and access them.

Ruby

regex = /(?<[1]>\d{4})-(?<[2]>\d{2})-(?<[3]>\d{2})/
match = regex.match('2023-06-15')
year = match[:year]
month = match[:month]
day = match[:day]

Drag options to blanks, or click blank then click option'

Ayear

Bmonth

Cday

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect or inconsistent capture group names.

Not matching the capture group names with the symbols used to access them.

The regex uses named captures 'year', 'month', and 'day' to extract parts of the date string.

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