Explain Regular Expressions in JavaScript. Provide examples of common patterns and use cases.

Explain Regular Expressions in JavaScript. Provide examples of common patterns and use cases.

Regular expressions are patterns used to match character combinations in strings.

Basic Syntax:

// Literal notation
const regex = /pattern/flags;

// Constructor notation
const regex = new RegExp('pattern', 'flags');

// Common flags
const regex = /hello/gi; // g = global, i = case-insensitive

Common Patterns:

// Email validation
const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
console.log(emailRegex.test('user@example.com')); // true

// Phone number (US format)
const phoneRegex = /^\(\d{3}\) \d{3}-\d{4}$/;
console.log(phoneRegex.test('(555) 123-4567')); // true

// URL validation
const urlRegex = /^https?:\/\/[\w\-]+(\.[\w\-]+)+([\w\-\.,@?^=%&:\/~\+#]*[\w\-\@?^=%&\/~\+#])?$/;
console.log(urlRegex.test('https://www.example.com')); // true

// Password (8+ chars, 1 uppercase, 1 lowercase, 1 number)
const passwordRegex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[a-zA-Z\d@$!%*?&]{8,}$/;
console.log(passwordRegex.test('Password123')); // true

String Methods:

const text = 'Hello World! Hello JavaScript!';

// test() - returns boolean
const regex = /hello/i;
console.log(regex.test(text)); // true

// exec() - returns match details or null
const match = regex.exec(text);
console.log(match); // ['Hello', index: 0, input: 'Hello World! Hello JavaScript!']

// match() - returns array of matches
const matches = text.match(/hello/gi);
console.log(matches); // ['Hello', 'Hello']

// replace() - replaces matches
const newText = text.replace(/hello/gi, 'Hi');
console.log(newText); // 'Hi World! Hi JavaScript!'

// search() - returns index of first match
const index = text.search(/world/i);
console.log(index); // 6

// split() - splits string by pattern
const words = text.split(/\s+/);
console.log(words); // ['Hello', 'World!', 'Hello', 'JavaScript!']

Common Use Cases:

// Extract data from strings
const text = 'Contact: John Doe (555) 123-4567, jane@example.com';
const phoneRegex = /\(\d{3}\) \d{3}-\d{4}/g;
const emailRegex = /[^\s@]+@[^\s@]+\.[^\s@]+/g;

const phones = text.match(phoneRegex); // ['(555) 123-4567']
const emails = text.match(emailRegex); // ['jane@example.com']

// Format strings
function formatPhoneNumber(phone) {
  const cleaned = phone.replace(/\D/g, ''); // Remove non-digits
  const match = cleaned.match(/^(\d{3})(\d{3})(\d{4})$/);
  if (match) {
    return `(${match[1]}) ${match[2]}-${match[3]}`;
  }
  return phone;
}

console.log(formatPhoneNumber('5551234567')); // '(555) 123-4567'

// Validate and sanitize input
function sanitizeInput(input) {
  return input
    .replace(/<script\b[^<]*(?:(?!<\/script>)<[^<]*)*<\/script>/gi, '') // Remove script tags
    .replace(/[<>]/g, ''); // Remove angle brackets
}

// Extract URLs from text
function extractUrls(text) {
  const urlRegex = /https?:\/\/[^\s]+/g;
  return text.match(urlRegex) || [];
}