Common patterns and character classes in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the string contains only digits.

Ruby

if str =~ /\A[1]+\z/
  puts "Only digits"
else
  puts "Contains other characters"
end

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

A\d

B\w

C\s

D\D

Attempts:

3 left

💡 Hint

Common Mistakes

Using \w matches letters and digits, not only digits.

Using \D matches non-digit characters, which is opposite of what is needed.

The pattern \d matches any digit character. Using \A\d+\z ensures the entire string is digits only.

2fill in blank

medium

Complete the code to find all whitespace characters in the string.

Ruby

matches = str.scan(/[1]/)
puts matches

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

A\S

B\s

C\w

D\d

Attempts:

3 left

💡 Hint

Common Mistakes

Using \S which matches non-whitespace characters.

Using \w which matches letters and digits, not spaces.

The pattern \s matches any whitespace character like spaces, tabs, or newlines.

3fill in blank

hard

Fix the error in the regex to match a string that starts with a letter.

Ruby

if str =~ /^[1]/i
  puts "Starts with a letter"
else
  puts "Does not start with a letter"
end

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

A\s

B[0-9]

C\d

D[a-z]

Attempts:

3 left

💡 Hint

Common Mistakes

Using \d which matches digits, not letters.

Using [0-9] which matches digits, not letters.

The pattern [a-z] matches any letter from a to z. The i flag makes it case-insensitive.

4fill in blank

hard

Fill both blanks to create a hash with words as keys and their lengths as values, only for words longer than 3 characters.

Ruby

lengths = words.each_with_object({}) { |word, h| h[word] = word.[1] if word.[2] 3 }

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

Alength

Bsize

Clength >

Dsize >

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' as a method without parentheses in this context.

Using 'length >' or 'size >' as one option instead of separating method and operator.

In Ruby, word.size returns the length of the word. The condition word.size > 3 filters words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a hash with uppercase words as keys and their lengths as values, only for words containing digits.

Ruby

result = words.each_with_object({}) { |word, h| h[word.[1]] = word.[2] if word =~ /[3]/ }

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

Aupcase

Bsize

C\d

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' instead of 'size' inconsistently.

Using incorrect regex like /\w/ which matches letters and digits.

word.upcase converts the word to uppercase for the key. word.size gives the length for the value. The regex /\d/ checks if the word contains any digit.

Practice

1. Which Ruby regular expression pattern matches any single digit character?

easy

A. /\D/

B. /\d/

C. /\s/

D. /\w/

Solution

1. Step 1: Understand \d in Ruby regex

   The pattern \d matches any digit character (0-9).

2. Step 2: Compare with other options

   \w matches word characters (letters, digits, underscore), \s matches whitespace, \D matches non-digit characters.

3. Final Answer:

   /\d/ -> Option B

4. Quick Check:

   Digit pattern = /\d/ [OK]

Hint: Remember \d means digit, \w means word character [OK]

Common Mistakes:

• Confusing \d with \w
• Using /\D/ which matches non-digits
• Thinking \s matches digits

2. Which of the following is the correct Ruby syntax to match any word character one or more times?

easy

A. /\w+/

B. /\w*/

C. /\w?/

D. /\w{}/

Solution

1. Step 1: Understand quantifiers in regex

   + means one or more times, * means zero or more, ? means zero or one.

2. Step 2: Apply to \w

   /\w+/ matches one or more word characters, which is the correct syntax.

3. Final Answer:

   /\w+/ -> Option A

4. Quick Check:

   One or more word chars = /\w+/ [OK]

Hint: Use + for one or more matches in regex [OK]

Common Mistakes:

• Using * which allows zero matches
• Using ? which allows zero or one match
• Using empty braces {} which is invalid

3. What is the output of this Ruby code?

text = "Room 101"
match = text.match(/\d+/)
puts match[0]

medium

A. 101

B. Room

C. 1

D. nil

Solution

1. Step 1: Understand the regex /\d+/

   This pattern matches one or more digits in a row.

2. Step 2: Apply match to string "Room 101"

   The digits "101" are matched as a group, so match[0] returns "101".

3. Final Answer:

   101 -> Option A

4. Quick Check:

   Digits matched = 101 [OK]

Hint: \d+ grabs all digits in a row, not just one [OK]

Common Mistakes:

• Thinking it matches only one digit
• Confusing match[0] with match object
• Expecting the word part instead of digits

4. Find the error in this Ruby code snippet:

email = "[e-mail]"
if email =~ /\w+@\w+.\w+/
  puts "Valid email"
else
  puts "Invalid email"
end

medium

A. The regex should use \d instead of \w

B. The =~ operator is used incorrectly

C. The regex does not escape the dot properly

D. The string should be in single quotes

Solution

1. Step 1: Examine the regex pattern

   The pattern /\w+@\w+.\w+/ uses an unescaped . which matches any character, not a literal dot.

2. Step 2: Identify the error

   The dot after the domain must be escaped as \. to match the literal "." in email addresses.

3. Final Answer:

   The regex does not escape the dot properly -> Option C

4. Quick Check:

   Escape literal dot as \. [OK]

Hint: Remember to escape dot as \. to match literal dot [OK]

Common Mistakes:

• Thinking dot doesn't need escaping
• Misusing =~ operator
• Confusing \w with \d in email regex

5. You want to extract all words starting with a capital letter from the string "Hello World from Ruby". Which Ruby regex pattern should you use with scan?

hard

A. /\d+/

B. /\b[a-z]\w*/

C. /\s+/

D. /\b[A-Z]\w*/

Solution

1. Step 1: Understand the requirement

   We want words starting with a capital letter, so first letter must be A-Z.

2. Step 2: Analyze the regex options

   /\b[A-Z]\w*/ matches word boundary, then capital letter, then zero or more word chars.

3. Step 3: Check other options

   /\b[a-z]\w*/ matches lowercase start, /\d+/ matches digits, /\s+/ matches spaces, so they don't fit.

4. Final Answer:

   /\b[A-Z]\w*/ -> Option D

5. Quick Check:

   Capital-start words = /\b[A-Z]\w*/ [OK]

Hint: Use \b for word boundary and [A-Z] for capital letters [OK]

Common Mistakes:

• Using [a-z] instead of [A-Z]
• Forgetting word boundary \b
• Using digit or space patterns incorrectly