Common patterns and character classes in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby regex match?

Consider the following Ruby code snippet. What will be printed when it runs?

Ruby

text = "Email: [e-mail]"
match = text.match(/\b\w+@\w+\.\w+\b/)
puts match[0]

A Email

B nil

C example.com

D [e-mail]

💡 Hint

Look for the pattern that matches an email-like string.

The regex \b\w+@\w+\.\w+\b matches a word boundary, then one or more word characters, an '@', more word characters, a dot, and word characters again. It matches the email part in the string.

❓ Predict Output

intermediate

What does this regex find in the string?

Given the Ruby code below, what will be the output?

Ruby

text = "Call me at 123-456-7890 or 987-654-3210."
numbers = text.scan(/\d{3}-\d{3}-\d{4}/)
puts numbers.join(", ")

A Call, me, at

B 1234567890, 9876543210

C 123-456-7890, 987-654-3210

D []

💡 Hint

Look for the pattern matching phone numbers with dashes.

The regex \d{3}-\d{3}-\d{4} matches three digits, a dash, three digits, a dash, and four digits, which matches the phone numbers in the string.

🔧 Debug

advanced

Why does this regex fail to match?

The following Ruby code is intended to find all words starting with a capital letter. Why does it not find any matches?

Ruby

text = "I."
matches = text.scan(/[A-Z][a-z]+/)
puts matches

A It misses single-letter capital words like 'I' because the pattern requires at least one lowercase letter after the capital letter.

B The regex is case-insensitive by default, so it matches everything.

C The pattern uses wrong character classes; it should use \w instead of [a-z].

D The scan method returns nil if no matches are found.

💡 Hint

Check what the pattern requires after the capital letter.

The regex [A-Z][a-z]+ matches a capital letter followed by one or more lowercase letters. Single capital letters like 'I' have no lowercase letters after them, so they are not matched.

📝 Syntax

advanced

Which regex pattern correctly matches a hexadecimal color code?

You want to match strings like '#1a2b3c' or '#ABCDEF' in Ruby. Which pattern is correct?

A /#([0-9a-fA-F]{6})\b/

B /#([0-9g-zA-Z]{6})/

C /#([0-9a-f]{6})/i

D /#([0-9a-fA-F]{3,6})/

💡 Hint

Hex digits are 0-9 and a-f (case insensitive).

Option A matches a '#' followed by exactly 6 hex digits (0-9, a-f, A-F) and a word boundary. Option A is case-insensitive but misses the word boundary and is less precise. Option A includes invalid characters 'g-z'. Option A allows 3 to 6 digits, which is not strictly a 6-digit hex code.

🚀 Application

expert

How many matches does this regex find?

In Ruby, given the string and regex below, how many matches will be found?

Ruby

text = "abc123xyz456def789"
matches = text.scan(/[a-z]+\d+/)
puts matches.size

A 1

B 3

C 0

D 2

💡 Hint

Look for sequences of letters followed immediately by digits.

The regex [a-z]+\d+ matches one or more lowercase letters followed by one or more digits. The string has 'abc123', 'xyz456', and 'def789' matching this pattern, so 3 matches.

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