Unlock ES2018 Regex: Lookbehind, Named Groups, DotAll & Unicode Made Simple

Overview

If you have ever performed complex text processing with JavaScript, the ES2018 update brings four powerful regex enhancements that make such tasks easier and more expressive.

Lookbehind assertions

Named capturing groups

DotAll s flag

Unicode property classes

Lookahead and Lookbehind

Before ES2018 JavaScript only supported lookahead assertions ( (?=...) and (?!...)). ES2018 adds lookbehind assertions, written as (?<=...) for positive and (?<!...) for negative lookbehind.

const re = /(?<=€)\d+(\.\d*)?/;
console.log(re.exec('199'));   // → null (no preceding €)
console.log(re.exec('€199'));  // → ["199", undefined, index: 1, input: "€199", groups: undefined]

Multiple lookbehinds can be chained to create more complex patterns, e.g. matching a number of two digits that is not 35:

const re = /(?<=\d{2})(?<!35) meters/;
console.log(re.exec('35 meters')); // → null
console.log(re.exec('14 meters')); // → [" meters", index: 2, input: "14 meters", groups: undefined]

Named Capturing Groups

Named groups use the syntax (?<name>...). The match result contains a groups object where each property is the group name.

const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = re.exec('2020-03-04');
console.log(match.groups); // {year: "2020", month: "03", day: "04"}

If a named group does not participate in the match, its value is undefined.

DotAll s Flag

Normally the dot ( .) matches any character except line terminators ( \n and \r). Adding the s flag makes the dot match line terminators as well.

console.log(/./.test('
'));   // false
console.log(/./s.test('
'));  // true

The flag can be combined with any regex without altering previous behaviour.

Unicode Property Classes

With the u flag, Unicode property escapes \p{...} and \P{...} allow matching characters by their Unicode properties. For example, to match any Unicode digit:

const re = /\p{Number}/u;
console.log(re.test('𝟠')); // true

To match any alphabetic character:

const re = /\p{Alphabetic}/u;
console.log(re.test('漢')); // true

Negated property classes use \P{...} and match characters that do not have the specified property.

Compatibility

Browser support for these features varies; modern browsers and recent Node.js versions implement them fully. A compatibility table is often shown in documentation.

Summary

ES2018 extends JavaScript regular expressions with lookbehind assertions, named capturing groups, the dotAll s flag, and Unicode property classes. These additions make patterns more readable and powerful, especially for complex text processing tasks.

When writing intricate regexes, using a testing tool such as Regex101 or RegexBuddy helps visualise matches, debug syntax errors, and understand patterns written by others.