JavaScript RegExp Character Classes

Types of Character Classes

let pattern = /[abc]/;
console.log(pattern.test("apple")); // true

let pattern = /[^abc]/;
console.log(pattern.test("def")); // true

let pattern = /[a-z]/;
console.log(pattern.test("Hello")); // true

let pattern = /\d/; // digits
console.log(pattern.test("abc123")); // true

🔹 Basic Character Sets [abc]

Square brackets define a character set - matches any single character inside:

// Match any vowel
let vowels = /[aeiou]/;
console.log(vowels.test("hello"));    // true (matches 'e')
console.log(vowels.test("world"));    // true (matches 'o')
console.log(vowels.test("xyz"));      // false (no vowels)

// Match specific letters
let pattern = /[abc]/;
console.log(pattern.test("apple"));   // true (matches 'a')
console.log(pattern.test("banana"));  // true (matches 'b')
console.log(pattern.test("cherry"));  // true (matches 'c')
console.log(pattern.test("date"));    // false (no a, b, or c)

// Find all vowels in text
let text = "Hello World";
let allVowels = text.match(/[aeiou]/gi);
console.log(allVowels); // ["e", "o", "o"]

🔹 Negated Character Sets [^abc]

The caret (^) inside brackets means "NOT" - matches any character except those listed:

// Match any character that is NOT a vowel
let notVowels = /[^aeiou]/;
console.log(notVowels.test("hello"));  // true (matches 'h')
console.log(notVowels.test("aeiou"));  // false (all are vowels)

// Match any character that is NOT a digit
let notDigits = /[^0-9]/;
console.log(notDigits.test("123"));    // false (all are digits)
console.log(notDigits.test("abc"));    // true (matches 'a')

// Remove all vowels from text
let text = "Hello World";
let consonantsOnly = text.replace(/[aeiou]/gi, "");
console.log(consonantsOnly); // "Hll Wrld"

// Keep only letters (remove numbers and symbols)
let mixedText = "Hello123World!";
let lettersOnly = mixedText.replace(/[^a-zA-Z]/g, "");
console.log(lettersOnly); // "HelloWorld"

🔹 Character Ranges [a-z]

Use hyphens to specify ranges of characters:

// Match any lowercase letter
let lowercase = /[a-z]/;
console.log(lowercase.test("Hello")); // true (matches 'e', 'l', 'l', 'o')
console.log(lowercase.test("HELLO")); // false (no lowercase)

// Match any uppercase letter
let uppercase = /[A-Z]/;
console.log(uppercase.test("Hello")); // true (matches 'H')

// Match any letter (upper or lower)
let anyLetter = /[a-zA-Z]/;
console.log(anyLetter.test("123"));   // false
console.log(anyLetter.test("a1"));    // true (matches 'a')

// Match any digit
let digit = /[0-9]/;
console.log(digit.test("abc"));       // false
console.log(digit.test("a1b"));       // true (matches '1')

// Combine ranges
let alphanumeric = /[a-zA-Z0-9]/;
console.log(alphanumeric.test("@#$")); // false
console.log(alphanumeric.test("a1@")); // true (matches 'a' or '1')

// Extract all letters and numbers
let text = "Hello123World!@#";
let alphanumericChars = text.match(/[a-zA-Z0-9]/g);
console.log(alphanumericChars); // ["H","e","l","l","o","1","2","3","W","o","r","l","d"]

🔹 Predefined Character Classes

JavaScript provides shortcuts for common character classes:

// \d - Match digits
let digits = /\d/;
console.log(digits.test("abc123")); // true
console.log("Price: $25.99".match(/\d+/g)); // ["25", "99"]

// \w - Match word characters
let word = /\w/;
console.log(word.test("hello_123")); // true
console.log("[email protected]".match(/\w+/g)); // ["user", "email", "com"]

// \s - Match whitespace
let space = /\s/;
console.log(space.test("hello world")); // true
console.log("a b c".split(/\s+/)); // ["a", "b", "c"]

// Practical example: validate username
function isValidUsername(username) {
    // Only letters, numbers, and underscores, 3-20 characters
    let pattern = /^[a-zA-Z0-9_]{3,20}$/;
    return pattern.test(username);
}

console.log(isValidUsername("user123"));     // true
console.log(isValidUsername("user@name"));   // false (contains @)
console.log(isValidUsername("ab"));          // false (too short)

🔹 Practical Examples

Real-world applications of character classes:

🔸 Phone Number Validation

function formatPhoneNumber(phone) {
    // Remove all non-digits
    let cleaned = phone.replace(/[^\d]/g, '');
    
    // Check if it's 10 digits
    if (cleaned.length === 10) {
        return cleaned.replace(/(\d{3})(\d{3})(\d{4})/, '($1) $2-$3');
    }
    return "Invalid phone number";
}

console.log(formatPhoneNumber("1234567890"));     // "(123) 456-7890"
console.log(formatPhoneNumber("123-456-7890"));   // "(123) 456-7890"
console.log(formatPhoneNumber("(123) 456-7890")); // "(123) 456-7890"

🔸 Password Strength Check

function checkPasswordStrength(password) {
    let hasLower = /[a-z]/.test(password);
    let hasUpper = /[A-Z]/.test(password);
    let hasDigit = /[0-9]/.test(password);
    let hasSpecial = /[^a-zA-Z0-9]/.test(password);
    let isLongEnough = password.length >= 8;
    
    let strength = 0;
    if (hasLower) strength++;
    if (hasUpper) strength++;
    if (hasDigit) strength++;
    if (hasSpecial) strength++;
    if (isLongEnough) strength++;
    
    return {
        score: strength,
        level: strength < 3 ? 'Weak' : strength < 5 ? 'Medium' : 'Strong'
    };
}

console.log(checkPasswordStrength("password"));      // {score: 2, level: "Weak"}
console.log(checkPasswordStrength("Password123!"));  // {score: 5, level: "Strong"}

🔸 Extract Email Addresses

function extractEmails(text) {
    // Simple email pattern using character classes
    let emailPattern = /[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/g;
    return text.match(emailPattern) || [];
}

let text = "Contact us at [email protected] or [email protected]";
console.log(extractEmails(text)); // ["[email protected]", "[email protected]"]