Regular expressions in practice

Capturing groups

Round brackets group part of a pattern and also capture the text it matched, so you can use it later. Groups are numbered from 1 in the order of their opening bracket.

const m = '2026-10-19'.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(m[0]); // '2026-10-19' (the whole match)
console.log(m[1], m[2], m[3]); // '2026' '10' '19'

Named groups

const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const { year, month, day } = '2026-10-19'.match(re).groups;
console.log(`${day}/${month}/${year}`); // 19/10/2026

Replacing with captured parts

// $1, $2 … or $<name> refer to groups in the replacement string
console.log('2026-10-19'.replace(/(\d{4})-(\d{2})-(\d{2})/, '$3/$2/$1')); // '19/10/2026'

// a function gets the match and the groups and returns the replacement
const prices = 'tea 10, coffee 25';
console.log(prices.replace(/\d+/g, (n) => String(Math.round(n * 1.18)))); // 'tea 12, coffee 30'

Non-capturing groups and alternatives

(?:…) groups without capturing, which is useful with | or a quantifier when you do not need the text later: /(?:Mr|Ms|Dr)\.? [A-Z][a-z]+/ matches "Dr. Rao" or "Ms Iyer".

Greedy versus lazy

Quantifiers like * and + are greedy: they take as much text as possible and then give back only what is needed. Adding ? after them (*?, +?) makes them lazy: as little as possible.

const html = '<b>Price</b> and <b>Offer</b>';
console.log(html.match(/<b>.*<\/b>/)[0]);  // '<b>Price</b> and <b>Offer</b>' (greedy)
console.log(html.match(/<b>.*?<\/b>/)[0]); // '<b>Price</b>' (lazy)

Lookahead (in brief)

(?=…) checks that something follows, without including it in the match; (?!…) checks that it does NOT follow. A common use is password rules: /^(?=.*\d)(?=.*[A-Z]).{8,}$/ means "at least 8 characters, containing a digit and a capital letter".

Escaping special characters

These characters have special meaning: . * + ? ^ $ { } ( ) | [ ] \ /. To match them literally, put a backslash before them: \. matches a real dot. When you build a regex from user input with new RegExp, escape the input first, or a user typing "(" will break it. ES2025 added RegExp.escape for this; in older environments use text.replace(/[.*+?^${}()|[\]\\]/g, '\\$&').

Unicode and Indian languages

Without the u flag, some characters (like many emoji) count as two units, and \w only matches English letters. With u you can use Unicode property escapes: /\p{Script=Devanagari}+/u matches Hindi words.

console.log('नमस्ते India'.match(/\p{Script=Devanagari}+/u)[0]); // 'नमस्ते'