Regular Expressions in JavaScript

const regex = /hello/;
const regex2 = /hello/i; // Case-insensitive
const regex3 = /hello/g; // Global
const regex4 = /hello/gi; // Global and case-insensitive

const regex = new RegExp("hello");
const regex2 = new RegExp("hello", "i");
const regex3 = new RegExp("hello", "gi");

const regex = /hello/;
console.log(regex.test("hello world")); // true
console.log(regex.test("Hello world")); // false

const regex1 = /[abc]/; // Matches a, b, or c
const regex2 = /[a-z]/; // Matches any lowercase letter
const regex3 = /[0-9]/; // Matches any digit
const regex4 = /[^abc]/; // Matches anything except a, b, c

const regex1 = /\d/; // Digit [0-9]
const regex2 = /\D/; // Non-digit [^0-9]
const regex3 = /\w/; // Word character [a-zA-Z0-9_]
const regex4 = /\W/; // Non-word character
const regex5 = /\s/; // Whitespace
const regex6 = /\S/; // Non-whitespace

const regex1 = /a*/; // 0 or more
const regex2 = /a+/; // 1 or more
const regex3 = /a?/; // 0 or 1
const regex4 = /a{3}/; // Exactly 3
const regex5 = /a{2,4}/; // 2 to 4
const regex6 = /a{2,}/; // 2 or more

const regex1 = /^hello/; // Starts with hello
const regex2 = /world$/; // Ends with world
const regex3 = /^hello world$/; // Exact match

const regex = /hello/i;
console.log(regex.test("Hello World")); // true
console.log(regex.test("Goodbye")); // false

const str = "The numbers are 123 and 456";
const matches = str.match(/\d+/g);
console.log(matches); // ["123", "456"]

const str = "Hello World";
console.log(str.search(/world/i)); // 6
console.log(str.search(/xyz/)); // -1

const str = "Hello World";
console.log(str.replace(/world/i, "JavaScript")); // "Hello JavaScript"
console.log(str.replace(/l/g, "L")); // "HeLLo WorLd"

const str = "apple apple apple";
console.log(str.replaceAll("apple", "orange")); // "orange orange orange"

const str = "apple,banana;orange:grape";
const fruits = str.split(/[,;:]/);
console.log(fruits); // ["apple", "banana", "orange", "grape"]

function isValidEmail(email) {
    const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
    return regex.test(email);
}

console.log(isValidEmail("[email protected]")); // true
console.log(isValidEmail("invalid.email")); // false

function isValidPhone(phone) {
    const regex = /^\d{3}-\d{3}-\d{4}$/;
    return regex.test(phone);
}

console.log(isValidPhone("123-456-7890")); // true
console.log(isValidPhone("1234567890")); // false

function isValidURL(url) {
    const regex = /^https?:\/\/.+\..+/;
    return regex.test(url);
}

console.log(isValidURL("https://example.com")); // true
console.log(isValidURL("not a url")); // false

function extractDomain(email) {
    const match = email.match(/@(.+)$/);
    return match ? match[1] : null;
}

console.log(extractDomain("[email protected]")); // "example.com"

function capitalizeWords(str) {
    return str.replace(/\b\w/g, char => char.toUpperCase());
}

console.log(capitalizeWords("hello world")); // "Hello World"

function stripHTML(html) {
    return html.replace(/<[^>]*>/g, "");
}

console.log(stripHTML("<p>Hello <b>World</b></p>")); // "Hello World"

function extractNumbers(str) {
    return str.match(/\d+/g).map(Number);
}

console.log(extractNumbers("I have 2 apples and 5 oranges")); // [2, 5]

function isStrongPassword(password) {
    const regex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/;
    return regex.test(password);
}

console.log(isStrongPassword("Weak")); // false
console.log(isStrongPassword("Strong@123")); // true

function generateSlug(title) {
    return title
        .toLowerCase()
        .trim()
        .replace(/\s+/g, "-")
        .replace(/[^\w-]/g, "");
}

console.log(generateSlug("Hello World!")); // "hello-world"

const regex = /(\d{3})-(\d{3})-(\d{4})/;
const match = "123-456-7890".match(regex);
console.log(match[1]); // "123"
console.log(match[2]); // "456"
console.log(match[3]); // "7890"

const regex = /(?:cat|dog)/;
const match = "I have a cat".match(regex);
console.log(match[0]); // "cat"

// Positive lookahead
const regex1 = /\d+(?=px)/;
console.log("100px".match(regex1)); // ["100"]

// Negative lookahead
const regex2 = /\d+(?!px)/;
console.log("100".match(regex2)); // ["100"]

// Good - specific pattern
const regex = /^[a-z]+@[a-z]+\.[a-z]+$/;

// Avoid - too broad
const regex2 = /.+@.+\..+/;

// Good - specific
const regex = /^[a-z]{3}$/;

// Avoid - excessive backtracking
const regex2 = /^[a-z]*[a-z]*[a-z]*$/;