How do you create a Regular Expression in JavaScript?

There are two ways: 1. Regex Literal Syntax: /pattern/flags (compiled at script load time, preferred); 2. RegExp Constructor: new RegExp('pattern', 'flags') (compiled at runtime, useful for dynamic user patterns).

What do the common regex flags 'g', 'i', and 'm' mean in JavaScript?

'g' enables global matching (finds all matches rather than stopping at the first match); 'i' enables case-insensitive matching; 'm' enables multi-line mode (affects ^ and $ anchors).

JavaScript Regular Expressions: Patterns, Methods & String Validation

A Regular Expression (RegEx) is a specialized sequence of characters defining a search pattern. In JavaScript, regular expressions are full first-class objects (RegExp) used for pattern matching, string searching, text parsing, and form data validation.

Creating Regular Expressions

// 1. Regular Expression Literal (Preferred)
const emailRegex = /^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/i;

// 2. RegExp Constructor (Used when pattern is dynamic)
const userInput = "javascript";
const dynamicPattern = new RegExp(userInput, "gi");

Core RegEx Methods: `test()` & `exec()`

Method	Owner	Description & Return Value
`regex.test(str)`	`RegExp`	Tests for match. Returns boolean (`true` or `false`). Fast for validation!
`regex.exec(str)`	`RegExp`	Executes search. Returns detailed match array with capture groups, or `null`.
`str.match(regex)`	`String`	Returns array of matches, or `null`.
`str.replace(regex, newStr)`	`String`	Replaces matching patterns with replacement string.

Essential Meta-Characters & Quantifiers Table

Symbol	Meaning	Example
`^` / `$`	Start of string / End of string anchors	`/^Hello$/`
`\d` / `\D`	Any digit [0-9] / Non-digit	`/\d{10}/` (10-digit phone)
`\w` / `\W`	Word character [a-zA-Z0-9_] / Non-word	`/\w+/`
`+` / `*` / `?`	1 or more / 0 or more / 0 or 1 (Optional)	`/https?/` (Matches http or https)
`{min, max}`	Range quantifier	`/\d{4,6}/`
`[abc]`	Character set (matches a, b, or c)	`/[aeiou]/` (Vowels)

// Practical Example: Extracting Indian 10-digit Phone Numbers
const text = "Contact our support at 9876543210 or alternate office desk 8765432109.";
const phoneRegex = /[6-9]\d{9}/g;

const matches = text.match(phoneRegex);
console.log("Discovered Phone Numbers:", matches); // ["9876543210", "8765432109"]

University Exam Corner (BCA Examination Notes)

Exam Question: "Write a JavaScript regular expression to validate a strong password (at least 8 characters, 1 uppercase, 1 lowercase, 1 number, and 1 special symbol)."

const strongPasswordPattern = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/;
console.log(strongPasswordPattern.test("VeewomPass@2026")); // true
console.log(strongPasswordPattern.test("weakpass"));         // false

Next Step

Need a rapid cheat sheet covering all syntax, methods, and keyboard shortcuts? Proceed to the final master resource: JavaScript Quick Reference Cheat Sheet →