Regular Expressions in JavaScript: Searching and Validating Text Patterns

// 😱 Validating an email WITHOUT regex — the naive approach:function isEmail(text) {
return text.includes("@") && // has @         text.includes(".") &&           // has a dot         text.indexOf("@") > 0 &&        // @ isn't first         text.indexOf("@") === text.lastIndexOf("@") &&  // only one @         text.indexOf(".") > text.indexOf("@") + 1 &&    // dot after @         !text.endsWith(".") &&          // doesn't end with dot         !text.includes(" ");            // no spaces  // …and this STILL accepts garbage like "a@.b" 😵
}

const emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
console.log(emailPattern.test("priya@example.com"));
// true ✅console.log(emailPattern.test("not-an-email"));        // false ❌console.log(emailPattern.test("a@b"));                 // false ❌

// ① Literal syntax (the everyday way) — pattern between slashes:const pattern1 = /hello/;
// ② Constructor syntax — for dynamically built patterns:const pattern2 = new RegExp("hello");
// Both work the same:console.log(pattern1.test("hello world"));   // trueconsole.log(pattern2.test("hello world"));   // true

console.log(/cat/.test("I love my CAT"));
// false — case sensitive!console.log(/cat/i.test("I love my CAT"));    // true — i flag ignores case ✅

const hasDigits = /\d/;
// \d = any digit
console.log(hasDigits.test("abc"));
// false — no digitsconsole.log(hasDigits.test("abc123"));   // true ✅console.log(hasDigits.test("42"));       // true ✅

const sentence = "I have 2 cats and 3 dogs";
console.log(sentence.match(/\d/));
// ["2"] — first match onlyconsole.log(sentence.match(/\d/g));     // ["2", "3"] — g flag finds ALL ✅console.log(sentence.match(/\d+/g));    // ["2", "3"] — + means "one or more"console.log(sentence.match(/zebra/));   // null — no match found

console.log(/\d\d\d/.test("Call 911 now"));
// true — three digits in a rowconsole.log(/\s/.test("hello world"));        // true — contains a space

console.log(/[aeiou]/.test("sky"));
// false — no vowelsconsole.log(/[aeiou]/.test("say"));       // true ✅console.log(/[^0-9]/.test("123a"));       // true — "a" is not a digit

// PIN code: exactly 6 digitsconst pinPattern = /^\d{
6
}$/;
console.log(pinPattern.test("400001"));
// true ✅console.log(pinPattern.test("40001"));     // false — only 5console.log(pinPattern.test("4000012"));   // false — 7 is too many

/^hello/.test("hello world");
// true — starts with hello/^hello/.test("say hello");      // false — hello isn't at the start/world$/.test("hello world");    // true — ends with world ✅
// ^...$ together = "the ENTIRE string must match":/^\d{3}$/.test("123");     // true ✅/^\d{3}$/.test("1234");    // false — extra digit!/^\d{3}$/.test("a123");    // false — extra letter!

const emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
// Reading it left to right:// ^           start// [^\s@]+     one or more chars that are NOT space or @  → the username// @           a literal @// [^\s@]+     one or more non-space/non-@ chars          → the domain// \.          a literal dot (escaped — plain . means "any char"!)// [^\s@]+     one or more chars                          → the extension// $           end
console.log(emailPattern.test("priya@example.com"));
// true ✅console.log(emailPattern.test("priya@example"));       // false — no .comconsole.log(emailPattern.test("priya example.com"));   // false — no @console.log(emailPattern.test("@example.com"));        // false — empty username

// 10 digits, starting with 6, 7, 8, or 9:const mobilePattern = /^[6-9]\d{9}$/;
console.log(mobilePattern.test("9876543210"));
// true ✅console.log(mobilePattern.test("1234567890"));   // false — starts with 1console.log(mobilePattern.test("98765"));        // false — too short

// At least 8 chars, one uppercase, one lowercase, one digit:const strongPassword = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/;
console.log(strongPassword.test("Passw0rd"));
// true ✅console.log(strongPassword.test("password"));   // false — no uppercase/digitconsole.log(strongPassword.test("PASS123"));    // false — no lowercaseconsole.log(strongPassword.test("Pass1"));      // false — too short

// 3–15 chars: letters, digits, underscore; must start with a letter:const usernamePattern = /^[A-Za-z]\w{2,14}$/;
console.log(usernamePattern.test("priya_dev"));
// true ✅console.log(usernamePattern.test("1st_place"));   // false — starts with digitconsole.log(usernamePattern.test("ab"));          // false — too short

// Censor numbers:"I paid 500 for 2 tickets".replace(/\d+/g, "X");// "I paid X for X tickets"
// Collapse multiple spaces into one:"too    many     spaces".replace(/\s+/g, " ");// "too many spaces"
// Reformat a date with CAPTURE GROUPS (parentheses!):"2026-09-19".replace(/(\d{4})-(\d{2})-(\d{2})/, "$3/$2/$1");// "19/09/2026" — $1, $2, $3 reuse the captured groups ✨

const url = "https://example.com/shop";
const parts = url.match(/^(https?):\/\/([^/]+)(\/.*)?$/);
console.log(parts[1]);
// "https"        — protocol
console.log(parts[2]);
// "example.com"  — domain
console.log(parts[3]);
// "/shop"        — path

// 📝 Complete signup validation — everything from this lesson!
function validateSignup( {
  username, email, mobile, password
}) {
  const rules = {
    username: /^[A-Za-z]\w {
      2,14
    }$/, email: /^[^\s@]+@[^\s@]+\.[^\s@]+$/, mobile: /^[6-9]\d {
      9
    }$/, password: /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d). {
      8,
    }$/
  };
  const messages = {
    username: "3–15 letters/digits/_, must start with a letter", email: "Enter a valid email (e.g., name@example.com)", mobile: "10-digit number starting with 6–9", password: "Min 8 chars with uppercase, lowercase & a digit"
  };
  let allValid = true;
  for (let field in rules) {
    // for…in from Lesson 17! 🗝️
    if (rules[field].test(field === undefined ? "" : eval(""))) {
    } if (!rules[field].test(eval(field) || "")) {
    }
  } // Cleaner explicit version:
  for (let field in rules) {
    const value = {
      username, email, mobile, password
    }[field] ?? "";
    if (rules[field].test(value)) {
      console.log(`✅ ${field}: valid`);
    } else {
      console.log(`❌ ${field}: ${messages[field]}`);
      allValid = false;
    }
  } return allValid;
}validateSignup( {
  username: "priya_dev", email: "priya@example.com", mobile: "9876543210", password: "Passw0rd"
});

✅ username: valid✅ email: valid✅ mobile: valid✅ password: valid