Scan for all matches in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Ruby method String#scan do?

It searches the string for all occurrences that match a given pattern and returns them as an array.

beginner

How do you use scan to find all words in a string?

Use scan with a regular expression like /\w+/ to find all word sequences.

intermediate

What is the return type of scan when used with a pattern that has capture groups?

It returns an array of arrays, where each inner array contains the captured groups for each match.

intermediate

How can you use a block with scan?

You can pass a block to scan to process each match as it is found instead of collecting them in an array.

beginner

What happens if scan finds no matches?

It returns an empty array.

What does "hello123".scan(/\d+/) return?

A ["h", "e", "l", "l", "o"]

B ["hello"]

C []

D ["123"]

The pattern /\d+/ matches one or more digits, so it finds "123".

Which of these returns an array of arrays when using scan?

A Using a pattern without capture groups

B Using a pattern with capture groups

C Using <code>scan</code> with no arguments

D Using <code>scan</code> on an empty string

Capture groups cause scan to return arrays of the captured parts.

What does "abc123def".scan(/[a-z]+/) return?

A ["abc", "def"]

B ["abc123def"]

C ["123"]

D []

The pattern matches sequences of letters, so it finds "abc" and "def".

How can you process matches one by one with scan?

A By passing a block to <code>scan</code>

B By calling <code>each</code> on the string

C By using <code>match</code> instead

D By converting the string to an array first

Passing a block to scan lets you handle each match as it is found.

What does "no digits".scan(/\d+/) return?

A ["0"]

B ["no", "digits"]

C []

D nil

No digits are found, so scan returns an empty array.

Explain how the Ruby scan method works to find all matches in a string.

Think about how you find all parts of a text that fit a pattern.

Describe how you can use a block with scan and why it might be useful.

Consider what happens if you want to do something with each match immediately.

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