Regular expressions

Write a function that tests whether a string is a valid email address (contains @, a domain, and a dot in the domain part).

function isValidEmail(str) {
    // your code here
}

console.log(isValidEmail('[email protected]'));   // true
console.log(isValidEmail('user@example'));        // false
console.log(isValidEmail('notanemail'));          // false
console.log(isValidEmail('[email protected]'));              // true

function isValidEmail(str) {
    return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(str);
}

Extract all words from a string that start with a capital letter. Return them as an array.

function capitalWords(str) {
    // your code here
}

console.log(capitalWords('Hello world, My Name is Alice'));  // ['Hello', 'My', 'Name', 'Alice']
console.log(capitalWords('no capitals here'));               // []

function capitalWords(str) {
    return str.match(/\b[A-Z][a-zA-Z]*/g) || [];
}

Replace all sequences of whitespace (spaces, tabs, newlines) in a string with a single space, and trim the result.

function normalizeSpaces(str) {
    // your code here
}

console.log(normalizeSpaces('  hello   world  '));         // 'hello world'
console.log(normalizeSpaces('line1\n\nline2\t  line3')); // 'line1 line2 line3'

function normalizeSpaces(str) {
    return str.replace(/\s+/g, ' ').trim();
}

Extract all numbers (integers and decimals) from a string and return them as an array of numbers.

function extractNumbers(str) {
    // your code here
}

console.log(extractNumbers('Price: 12.5, qty: 3, discount: 0.15'));  // [12.5, 3, 0.15]
console.log(extractNumbers('no numbers here'));                       // []

function extractNumbers(str) {
    const matches = str.match(/\d+\.?\d*/g);
    return matches ? matches.map(Number) : [];
}

Use a capturing group to extract the username and domain from an email address string.

function parseEmail(email) {
    // Return { username, domain } or null if not valid
}

console.log(parseEmail('[email protected]'));   // { username: 'user', domain: 'example.com' }
console.log(parseEmail('notanemail'));          // null

function parseEmail(email) {
    const match = email.match(/^([^\s@]+)@([^\s@]+)$/);
    if (!match) return null;
    return { username: match[1], domain: match[2] };
}

Write a function that converts a camelCase string to snake_case (e.g., 'myVariableName' → 'my_variable_name').

function camelToSnake(str) {
    // your code here
}

console.log(camelToSnake('myVariableName'));    // 'my_variable_name'
console.log(camelToSnake('helloWorld'));        // 'hello_world'
console.log(camelToSnake('alreadylower'));      // 'alreadylower'

function camelToSnake(str) {
    return str.replace(/([A-Z])/g, '_$1').toLowerCase();
}

Count how many times a word appears in a string (case-insensitive, whole words only).

function countWord(str, word) {
    // your code here
}

console.log(countWord('The cat sat on the mat. The cat is fat.', 'the'));  // 3
console.log(countWord('cats and cat and cat', 'cat'));                      // 3 (not 'cats')

function countWord(str, word) {
    const escaped = word.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
    const re = new RegExp(`\\b${escaped}\\b`, 'gi');
    return (str.match(re) || []).length;
}

Split a string of comma/semicolon/space-separated values into an array of trimmed non-empty tokens.

function splitTokens(str) {
    // your code here
}

console.log(splitTokens('a, b;  c , d'));    // ['a', 'b', 'c', 'd']
console.log(splitTokens('one;two,three'));   // ['one', 'two', 'three']
console.log(splitTokens('  , ; '));          // []

function splitTokens(str) {
    return str.split(/[,;\s]+/).map(t => t.trim()).filter(Boolean);
}

Use regex to mask all but the last four digits of a credit card number (replace with '*').

function maskCard(number) {
    // your code here
}

console.log(maskCard('1234567890123456'));  // '************3456'
console.log(maskCard('4111 1111 1111 1111')); // '**** **** **** 1111'

function maskCard(number) {
    return number.replace(/\d(?=\d{4})/g, '*');
}

Extract all URLs from a string. A URL starts with http:// or https:// and ends at whitespace or end of string.

function extractUrls(str) {
    // your code here
}

const text = 'Visit https://example.com and http://foo.org/path?q=1 for details.';
console.log(extractUrls(text));
// ['https://example.com', 'http://foo.org/path?q=1']

function extractUrls(str) {
    return str.match(/https?:\/\/[^\s]+/g) || [];
}