For years, JavaScript developers envied Python and PCRE for their "lookbehind" capabilities. With ES2018, that wait ended. You can now match a pattern only if it is preceded by another pattern.

Positive Lookbehind (?<=...)

This asserts that what precedes the current position is the given pattern, but it doesn't consume characters.

// Match a price amount, but not the dollar sign
const price = "$500";
const regex = /(?<=$)\d+/; 
console.log(price.match(regex)); // Output: ["500"]

Negative Lookbehind (?<!...)

Ensures the pattern is not preceded by a specific term.

/(?<!not )feasible/

This matches "feasible" only if the word "not " is not immediately before it.

Browser Compatibility

Support is widespread in modern Node.js (8.10+), Chrome, and Firefox. However, always transpile with Babel if you need to support Internet Explorer (RIP) or very old Safari versions.