Regex Character Classes and Ranges Essentials Quiz

Explore the fundamentals of regular expression character classes and ranges with targeted, scenario-based questions. Strengthen your understanding of regex syntax and improve your ability to build flexible patterns for string matching tasks.

1. Understanding Basic Character Classes

   Which regex pattern matches any single lowercase letter from 'a' to 'z' in the word 'apple'?

   1. [a-z]
   2. [A-Z]
   3. (a-z)
   4. {a-z}

   Explanation: [a-z] defines a character class that matches any single lowercase letter between a and z, which applies to the word 'apple'. [A-Z] is incorrect because it matches only uppercase letters. (a-z) is not a valid regex syntax and would not function as a character class. {a-z} is also incorrect since curly brackets are used for quantifiers, not character classes.

2. Using Negated Character Classes

   What does the regex pattern [^0-9] match in the string 'Room42'?

   1. Any character that is not a digit
   2. Any digit between 0 and 9
   3. A dash or a caret
   4. Only alphabetic letters

   Explanation: [^0-9] defines a negated character class and matches any character that is not a digit, so it would match 'R', 'o', 'o', and 'm' in 'Room42'. [0-9] would match only digits, making that option incorrect. A dash or a caret is not relevant here since they are used as special characters in regex patterns. Only alphabetic letters is too restrictive because the character class could also match symbols or spaces.

3. Combining Multiple Ranges

   Which character class matches any uppercase or lowercase English letter in the pattern?

   1. [A-Za-z]
   2. [a-zA-Z0-9]
   3. [a-z]
   4. [A-Z0-9]

   Explanation: [A-Za-z] combines both uppercase and lowercase letter ranges, ensuring a match with any English letter. [a-zA-Z0-9] includes digits, so it is broader than needed. [a-z] only matches lowercase letters, and [A-Z0-9] matches only uppercase letters and digits, which makes both options incomplete for this scenario.

4. Special Character Classes

   In regex, what does the character class [dw] match?

   1. Any digit or any word character
   2. Only whitespace characters
   3. Any non-digit character
   4. Only letters, not digits

   Explanation: [dw] matches either any digit (d) or any word character (w), where word characters include letters, digits, and underscores. Only whitespace characters are matched by the s class, so that option is incorrect. Any non-digit character would require a caret, such as [^d]. Matching only letters, not digits, would exclude digits, but w includes digits by definition.

5. Correct Use of Dash Inside Character Classes

   How does the regex pattern [a-z-] differ from [a-z]?

   1. It matches any lowercase letter or a hyphen character
   2. It matches uppercase and lowercase letters
   3. It only matches the hyphen character
   4. It ignores the hyphen and matches only letters

   Explanation: [a-z-] includes the range a to z and specifically adds the hyphen as a literal match, so it matches lowercase letters or hyphen characters. [a-z] excludes the hyphen, so option two is incorrect. Matching only the hyphen is wrong, as the range includes all lowercase letters as well. Option four is incorrect because the hyphen, when placed at the end or start inside the brackets, is considered a literal character.