Scan for all matches in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all words starting with 'a' in the string.

Ruby

text = "apple banana apricot cherry"
matches = text.scan([1])
puts matches

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

A/\ba\w*/

B/a/

C/^a/

D/a+/

Attempts:

3 left

💡 Hint

Common Mistakes

Using /a/ matches all 'a' characters, not whole words.

Using /^a/ matches only if 'a' is at the start of the string.

The regex /\ba\w*/ matches words starting with 'a'. The \b ensures word boundary.

2fill in blank

medium

Complete the code to find all numbers in the string.

Ruby

text = "There are 12 apples and 30 bananas."
numbers = text.scan([1])
puts numbers

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

A/\w+/

B/[a-z]+/

C/\d+/

D/\s+/

Attempts:

3 left

💡 Hint

Common Mistakes

Using /[a-z]+/ matches letters, not numbers.

Using /\s+/ matches spaces, not numbers.

The regex /\d+/ matches one or more digits, capturing all numbers.

3fill in blank

hard

Fix the error in the code to find all words ending with 'ing'.

Ruby

text = "I am singing and dancing"
matches = text.scan([1])
puts matches

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

A/\w+ing\b/

B/\bing/

C/ing$/

D/ing\b/

Attempts:

3 left

💡 Hint

Common Mistakes

Using /ing$/ matches only if 'ing' is at the end of the string.

Using /\bing/ matches words starting with 'ing', not ending.

The regex /\w+ing\b/ matches whole words ending with 'ing'.

4fill in blank

hard

Fill both blanks to find all words with exactly 4 letters.

Ruby

text = "This code finds four letter words like test and code"
matches = text.scan([1]).select { |word| word.length [2] 4 }
puts matches

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

A/\b\w+\b/

B==

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using >= or <= selects words longer or shorter than 4, not exactly 4.

Use /\b\w+\b/ to find words, then select those with length == 4.

5fill in blank

hard

Fill all three blanks to find all words starting with a vowel and longer than 3 letters.

Ruby

text = "apple orange banana egg umbrella cat dog"
matches = text.scan([1]).select { |word| word.length [2] 3 && word[0].downcase [3] /[aeiou]/ }
puts matches

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

A/\b\w+\b/

B>

C=~

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of =~ for regex match.

Using >= instead of > for length check.

Use /\b\w+\b/ to find words, select those longer than 3 and starting with a vowel using =~.

Practice

1. What does the Ruby method scan do when used on a string?

easy

A. It counts the number of characters in the string.

B. It replaces all matches of a pattern with another string.

C. It finds all matches of a pattern and returns them in an array.

D. It converts the string to uppercase.

Solution

1. Step 1: Understand the purpose of scan

   The scan method searches the string for all occurrences of a pattern.

2. Step 2: Identify the return value of scan

   It returns an array containing all matched parts of the string.

3. Final Answer:

   It finds all matches of a pattern and returns them in an array. -> Option C

4. Quick Check:

   scan returns matches array [OK]

Hint: Remember: scan returns all matches as an array [OK]

Common Mistakes:

• Confusing scan with gsub (replacement)
• Thinking scan returns a count
• Assuming scan modifies the original string

2. Which of the following is the correct syntax to find all digits in a string str using scan?

easy

A. str.scan(/d+)

B. str.scan("\\d+")

C. str.scan('\d+')

D. str.scan(/\d+/)

Solution

1. Step 1: Recognize the correct regex syntax in Ruby

   Ruby uses forward slashes /pattern/ to denote regular expressions.

2. Step 2: Identify the correct regex for digits

   \d+ matches one or more digits. The pattern must be inside slashes for regex.

3. Final Answer:

   str.scan(/\d+/) -> Option D

4. Quick Check:

   Regex in slashes with \d+ = str.scan(/\d+/) [OK]

Hint: Use /pattern/ for regex, not strings, in scan [OK]

Common Mistakes:

• Using string quotes instead of regex slashes
• Missing escape backslash in regex
• Using incorrect regex delimiters

3. What is the output of the following Ruby code?

text = "cat bat rat"
matches = text.scan(/.at/)
puts matches.inspect

medium

A. ["at", "at", "at"]

B. ["cat", "bat", "rat"]

C. ["c", "b", "r"]

D. []

Solution

1. Step 1: Understand the regex /.at/

   The dot . matches any single character, so .at matches any three-letter word ending with 'at'.

2. Step 2: Apply scan on the string

   The string "cat bat rat" contains "cat", "bat", and "rat" which all match the pattern.

3. Final Answer:

   ["cat", "bat", "rat"] -> Option B

4. Quick Check:

   scan finds all 3-letter words ending with 'at' [OK]

Hint: Dot matches any char; scan returns all full matches [OK]

Common Mistakes:

• Thinking dot matches zero or multiple chars
• Expecting partial matches like 'at' only
• Confusing scan output with string split

4. The following code is intended to find all words starting with 'a' in sentence. What is the error?

sentence = "apple banana apricot avocado"
words = sentence.scan("a\w+")
puts words.inspect

medium

A. The pattern should be a regex, not a string.

B. The pattern should use double backslashes.

C. The method scan does not work on strings.

D. The variable name 'words' is invalid.

Solution

1. Step 1: Check the argument type for scan

   The scan method expects a regex pattern, not a plain string.

2. Step 2: Identify correct regex usage

   The pattern should be written as /a\w+/ to match words starting with 'a'.

3. Final Answer:

   The pattern should be a regex, not a string. -> Option A

4. Quick Check:

   scan requires regex pattern, not string [OK]

Hint: Use /pattern/ for regex in scan, not string quotes [OK]

Common Mistakes:

• Passing string instead of regex to scan
• Misunderstanding backslash escaping
• Assuming scan modifies the string

5. Given the string data = "log12 ID123, ID456, ID789", how can you use scan to extract only the numeric parts after each 'ID' (e.g., ["123", "456", "789"])?

hard

A. data.scan(/ID(\d+)/).flatten

B. data.scan(/\d+/)

C. data.scan(/ID\d+/)

D. data.scan(/(ID\d+)/)

Solution

1. Step 1: Understand the pattern to extract numbers after 'ID'

   The regex /ID(\d+)/ matches 'ID' followed by digits, capturing only digits in parentheses. This ignores other digits like '12' in 'log12'.

2. Step 2: Use scan and flatten the result

   scan returns an array of arrays for capture groups, so flatten converts it to a simple array of strings.

3. Final Answer:

   data.scan(/ID(\d+)/).flatten -> Option A

4. Quick Check:

   Capture group extracts digits, flatten simplifies array [OK]

Hint: Use capture groups and flatten to get inner matches [OK]

Common Mistakes:

• Using data.scan(/\d+/) which grabs all digits including extras like '12' and '78'
• Using scan without flatten, getting nested arrays
• Matching full 'ID123' instead of just digits