Name: JavaScript Regular Expressions: Lookaheads and LookBehinds
Description: Lookaheads and lookbehinds are zero-width assertions that check what's before or after a position without consuming characters.

Lookaheads and lookbehinds are zero-width assertions that check what's before or after a position without consuming characters.

Lookaheads and lookbehinds are zero-width assertions that check what’s before or after a position without consuming characters. This lesson covers positive lookahead (?=…), negative lookahead, positive lookbehind, and negative lookbehind. You’ll build a real-world password validator that enforces uppercase letters, lowercase letters, digits, and a minimum length — all using nothing but lookaheads.

app.js

import output from "./output.js";

let str = `foo
foobar
foobaz
fooboo`;

let regex = /foo(bar)/g;
// lookahead asserts what's on the right
regex = /foo(?=bar|boo)/g; // is foo followed by bar?
regex = /foo(?!bar|boo)/g; // is foo NOT followed by bar?
// lookabehind asserts what's on the left
regex = /(?<=foo)(bar)/g;
regex = /(?<!foo)(bar)/g;

regex = /(?=.*[A-Z])(?=.*[a-z])(?=.*\d).{8,}/;
str = `Password1`;
console.log(regex.test(str));

output(str, regex);