Lesson 67 - Lookahead & Lookbehind Assertions in Regular Expressions

Lookahead and lookbehind assertions in regular expressions allow you to match a pattern based on what comes before or after it, without including those parts in the match. They are useful for complex pattern matching.

Lookahead Assertions

A lookahead assertion checks if a certain pattern appears after the current position without consuming it.

Positive Lookahead (?=...)

It matches a group only if it is followed by a certain pattern.

Example:

let text = "apple123 banana456 cherry789";
let regex = /\d+(?= banana)/g;
console.log(text.match(regex)); // ["123"]

Explanation: It finds numbers before "banana".

Negative Lookahead (?!...)

It ensures a pattern is not followed by a certain string.

Example:

let text = "apple123 banana456 cherry789";
let regex = /\d+(?! banana)/g;
console.log(text.match(regex)); // ["456", "789"]

Explanation: It matches numbers not followed by "banana".

Lookbehind Assertions

A lookbehind assertion checks if a certain pattern appears before the current position without consuming it.

Positive Lookbehind (?<=...)

It matches a group only if it is preceded by a certain pattern.

Example:

let text = "apple123 banana456 cherry789";
let regex = /(?<=banana)\d+/g;
console.log(text.match(regex)); // ["456"]

Explanation: It finds numbers after "banana".

Negative Lookbehind (?<!...)

It ensures a pattern is not preceded by a certain string.

Example:

let text = "apple123 banana456 cherry789";
let regex = /(?<!banana)\d+/g;
console.log(text.match(regex)); // ["123", "789"]

Explanation: It matches numbers not preceded by "banana".

Summary

Try It Yourself

Experiment with different lookaheads and lookbehinds on regex101.com.