JavaScript Regular Expressions (RegEx) Cheatsheet

JavaScript uses RegExp objects to work with regular expressions for searching and manipulating strings.

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🧠 Syntax Basics

const pattern = /expression/flags;
const regex = new RegExp("expression", "flags");

Flag	Meaning
g	Global search (all matches)
i	Case-insensitive
m	Multiline mode
s	Dot matches newline
u	Unicode
y	Sticky matching

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🔍 Common RegEx Patterns

1. Match an Email Address

const regex = /^[\w.-]+@[a-zA-Z\d.-]+\.[a-zA-Z]{2,}$/;

• Explanation: Matches user@domain.com
• \w = word char, . = dot, - = hyphen

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2. Match a URL

const regex = /https?:\/\/(www\.)?[a-zA-Z0-9-]+\.[a-zA-Z]{2,}(\/\S*)?/;

• Matches http://, https://, optional www, domain, and optional path

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3. Validate a Phone Number (US)

const regex = /^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$/;

• Matches formats like (123) 456-7890, 123-456-7890, 1234567890

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4. Match Only Digits

const regex = /^\d+$/;

• Only digits (0-9), at least one

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5. Match Only Letters

const regex = /^[a-zA-Z]+$/;

• Only alphabetic characters

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6. Check for Whitespace

const regex = /\s/;

• Matches any whitespace character

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7. Password Strength (8+ chars, upper, lower, number, special)

const regex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@#$%^&+=!]).{8,}$/;

• Uses lookaheads to ensure character variety

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8. Extract Numbers from a String

const regex = /\d+/g;
const numbers = "abc123def456".match(regex); // ['123', '456']

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9. Detect HTML Tags

const regex = /<[^>]*>/g;

• Matches any HTML tags like <div>, </p>, etc.

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10. Match Dates (MM/DD/YYYY)

const regex = /^(0[1-9]|1[0-2])\/(0[1-9]|[12]\d|3[01])\/\d{4}$/;

• Matches US-style dates like 04/16/2025

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11. Match Hex Colors

const regex = /^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/;

• Matches hex colors like #fff or #12a3ef

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12. Match Time (24-hour HH:MM)

const regex = /^([01]\d|2[0-3]):([0-5]\d)$/;

• Matches times like 14:30, 09:45

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🧪 Using RegEx in JavaScript

const pattern = /^\d{3}$/;
console.log(pattern.test("123")); // true
console.log("abc123".match(/\d+/g)); // ['123']

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📌 Tips

• Use RegExp.test() to check for a match (true/false)
• Use String.match() to extract matches
• Escape special characters like . or ? with \

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Let me know if you'd like this exported to a .md file or extended with advanced patterns (e.g., IPv4, slugs, currency formats).