Mastering Regex Lookahead and Lookbehind

Table of Contents

• What are Lookarounds?
• Positive Lookahead
• Negative Lookahead
• Positive Lookbehind
• Negative Lookbehind
• Real-World Examples

What are Lookarounds?

Lookaheads and lookbehinds are zero-width assertions — they check whether a pattern exists ahead/behind the current position without consuming characters.

(?=...)   - Positive lookahead
(?!...)   - Negative lookahead
(?<=...)  - Positive lookbehind
(?<!...)  - Negative lookbehind

Positive Lookahead

Match X only if followed by Y:

// Match digits followed by 'px'
"16px 2em 24px".match(/\d+(?=px)/g); // ["16", "24"]

Negative Lookahead

Match X only if NOT followed by Y:

// Match .js files but not .json
/\.js(?!on)/.test("app.js");    // true
/\.js(?!on)/.test("data.json"); // false

Positive Lookbehind

Match X only if preceded by Y (ES2018+):

// Match numbers preceded by "$"
"$100 EUR200 $300".match(/(?<=\$)\d+/g); // ["100", "300"]

Negative Lookbehind

Match X only if NOT preceded by Y:

// Match 'ing' not preceded by 'think'
/(?<!think)ing/.test("singing");  // true
/(?<!think)ing/.test("thinking"); // false

Real-World Examples

Password validation with lookaheads:

// At least 8 chars, one uppercase, one digit
/^(?=.*[A-Z])(?=.*\d).{8,}$/.test("Password1"); // true

Extract prices:

const prices = "$9.99 and EUR14.50";
prices.match(/(?<=[$])\d+\.\d{2}/g); // ["9.99"]

Test all lookahead/lookbehind patterns in the Regex Tester.