How to Use Regular Expressions in JavaScript (Practical Guide with Examples)

// Literal syntax (preferred when pattern is known)

const regex = /hello/i;

// Constructor syntax (for dynamic patterns)

const pattern = 'hello';

const regex = new RegExp(pattern, 'i');

Core JavaScript Regex Methods

test() — Returns true/false

const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$/;

emailRegex.test('user@example.com'); // true

emailRegex.test('not-an-email'); // false

match() — Returns matches array

const text = 'Prices: ₹500, ₹1200, ₹350';

const prices = text.match(/₹d+/g); // ['₹500', '₹1200', '₹350']

With the g flag, returns all matches. Without g, returns first match with groups.

replace() — Find and replace

// Remove all HTML tags

const clean = html.replace(/<[^>]+>/g, '');

// Format Indian phone numbers

const formatted = '9876543210'.replace(/(d{5})(d{5})/, '$1-$2');

// Result: '98765-43210'

replaceAll() — Replace all matches (ES2021+)

const text = 'foo bar foo';

text.replaceAll('foo', 'baz'); // 'baz bar baz'

split() — Split by pattern

// Split on any whitespace

'hello world tabs'.split(/s+/); // ['hello', 'world', 'tabs']

exec() — Detailed match with groups

const dateRegex = /(d{2})/(d{2})/(d{4})/;

const result = dateRegex.exec('Date: 15/03/2026');

// result[0]: '15/03/2026'

// result[1]: '15' (day)

// result[2]: '03' (month)

// result[3]: '2026' (year)

Regex Flags in JavaScript

|---|---|

/hello/gi.test('Hello World'); // true (case insensitive + global)

Named Capture Groups (ES2018+)

const dateRegex = /(?d{2})/(?d{2})/(?d{4})/;

const { day, month, year } = '15/03/2026'.match(dateRegex).groups;

// day: '15', month: '03', year: '2026'

Practical Patterns for Indian Development

Indian mobile number validation:

const isMobile = (num) => /^[6-9]d{9}$/.test(num.replace(/s|-/g, ''));

PAN card validation:

const isPAN = (pan) => /^[A-Z]{5}[0-9]{4}[A-Z]$/.test(pan.toUpperCase());

GST number validation:

const isGST = (gst) => /^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z][1-9A-Z]Z[0-9A-Z]$/.test(gst);

Aadhaar number (basic format check):

const isAadhaar = (num) => /^[2-9]{1}[0-9]{11}$/.test(num.replace(/s/g, ''));

PIN code validation:

const isPincode = (pin) => /^[1-9][0-9]{5}$/.test(pin);

Email validation (practical, covers 99.9% of real cases):

const isEmail = (email) => /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$/.test(email);

URL extraction from text:

Common Mistakes with JavaScript Regex

Forgetting to escape special characters in RegExp constructor:

// Wrong - dot matches any character

new RegExp('hello.world');

// Correct - escaped dot matches literal dot

new RegExp('hello\.world');

Forgetting the g flag for global replacement:

'aaa'.replace(/a/, 'b'); // 'baa' — only replaces first!

'aaa'.replace(/a/g, 'b'); // 'bbb' — replaces all

Testing regex against undefined:

// Safe pattern

const value = userInput ?? '';

if (/^d+$/.test(String(value))) { ... }