Regular Expressions

A regular expression ("regex") is a pattern for matching text. With one compact line you can check whether a string looks like a postcode, find every hashtag in a post, or reformat dates. The syntax looks cryptic at first, but a small core covers most practical needs.

Creating a regex

const literal = /cat/;
const fromString = new RegExp("cat");

console.log(literal.test("concatenate"));
console.log(fromString.test("dog"));

true
false

A regex literal is written between slashes. The RegExp constructor is useful when the pattern is built from a variable. test() returns true if the pattern appears anywhere in the string.

Flags

Letters after the closing slash change how matching works:

console.log(/hello/.test("Hello"));
console.log(/hello/i.test("Hello"));
console.log("a1b2c3".replace(/\d/, "#"));
console.log("a1b2c3".replace(/\d/g, "#"));

false
true
a#b2c3
a#b#c#

Character classes

Quantifiers and anchors

const zip = /^\d{5}(-\d{4})?$/;
console.log(zip.test("90210"), zip.test("90210-1234"), zip.test("9021"));

const hexColor = /^#([0-9a-f]{3}|[0-9a-f]{6})$/i;
console.log(hexColor.test("#D6336C"), hexColor.test("#fff"), hexColor.test("red"));

const username = /^[a-z][a-z0-9_]{2,15}$/;
console.log(username.test("ada_99"), username.test("9lives"));

true true false
true true false
true false

Without ^ and $, a pattern can match part of a string — /\d{5}/ would accept "abc123456xyz". Anchor validation patterns at both ends.

Finding matches

const post = "Loving #javascript and #webdev today! #js";

console.log(post.match(/#\w+/));
console.log(post.match(/#\w+/g));
console.log(post.search(/#/));

[
  '#javascript',
  index: 7,
  input: 'Loving #javascript and #webdev today! #js',
  groups: undefined
]
[ '#javascript', '#webdev', '#js' ]
7

Without g, match returns details of the first match (or null). With g, it returns an array of every matching string.

Capture groups

Parentheses capture part of a match. Named groups, written (?<name>…), make the result much more readable:

const date = "Due: 2026-10-06";
const m = date.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);

console.log(m.groups.year, m.groups.month, m.groups.day);
console.log(date.replace(/(\d{4})-(\d{2})-(\d{2})/, "$3/$2/$1"));

2026 10 06
Due: 06/10/2026

In a replacement string, $1, $2… refer to the captured groups.

Looping over all matches

const prices = "Tea $3.50, Cake $4.25, Juice $5";

for (const m of prices.matchAll(/(\w+) \$(\d+(?:\.\d+)?)/g)) {
  console.log(m[1], "→", Number(m[2]));
}

Tea → 3.5
Cake → 4.25
Juice → 5

(?:…) groups without capturing. matchAll requires the g flag and gives full details, including groups, for every match.

Replacing with a function

const text = "the quick brown fox";
console.log(text.replace(/\b\w/g, ch => ch.toUpperCase()));

const message = "Order 3 apples and 12 pears";
console.log(message.replace(/\d+/g, n => String(Number(n) * 2)));

The Quick Brown Fox
Order 6 apples and 24 pears

Escaping special characters

Characters like . * + ? ( ) [ ] { } ^ $ | \ / have special meanings. To match them literally, escape them with a backslash: /\$\d+\.\d{2}/ matches $4.99. When building a pattern from user input, escape it first — newer engines provide RegExp.escape() for exactly this.

Try it yourself