Common patterns and character classes in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the character class [a-z] represent in Ruby regular expressions?

It matches any single lowercase letter from a to z.

beginner

What does the pattern \d match in Ruby regular expressions?

It matches any single digit character, equivalent to [0-9].

intermediate

Explain the difference between \w and \W in Ruby regex.

\w matches any word character (letters, digits, and underscore), while \W matches any character that is NOT a word character.

beginner

What does the pattern \s match in Ruby regular expressions?

It matches any whitespace character, including spaces, tabs, and newline characters.

beginner

How do you match any character except a newline in Ruby regex?

Use the dot . character, which matches any character except newline by default.

Which pattern matches any uppercase letter in Ruby regex?

A \d

B [A-Z]

C [a-z]

D \s

The pattern [A-Z] matches any uppercase letter from A to Z.

What does the pattern \D match?

A Any non-digit

B Any digit

C Any whitespace

D Any word character

\D matches any character that is NOT a digit.

Which pattern matches a whitespace character?

A \d

B \w

C \s

D \W

\s matches any whitespace character like space, tab, or newline.

What does the pattern [^a-z] mean?

A Matches any lowercase letter

B Matches whitespace only

C Matches digits only

D Matches any character except lowercase letters

The caret ^ inside brackets negates the class, so it matches any character except lowercase letters.

Which pattern matches any word character?

A \w

B \D

C \s

D \W

\w matches letters, digits, and underscore characters.

Describe the common character classes used in Ruby regular expressions and give an example of each.

Think about digits, letters, and spaces.

Explain how to use negation inside character classes in Ruby regex and provide an example pattern.

Negation flips what the class 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