JavaScript Regular Expressions (Regex): Patterns, Flags, Groups and Methods

A regular expression (regex) is a pattern that describes text. With one short pattern you can check whether a string looks like a PIN code, find every hashtag in a post, or reformat dates. Regexes look cryptic at first, but they are built from a small number of pieces.

Creating and testing#

const pattern = /cat/;

console.log(pattern.test("The cat sat"));
console.log(pattern.test("The dog sat"));
console.log(/cat/i.test("CAT"));

true
false
true

• A regex literal is written between slashes: /cat/.
• regex.test(string) returns true if the pattern appears anywhere in the string.
• Letters after the closing slash are flags; i means ignore case.

You can also build a regex from a string with new RegExp("cat", "i"), which is useful when the pattern comes from a variable.

Character classes#

To match a character with a special meaning (such as ., ?, +, ( or /) literally, escape it with a backslash: \. matches a full stop.

Quantifiers#

console.log(/colou?r/.test("color"), /colou?r/.test("colour"));
console.log(/\d{3}-\d{4}/.test("call 555-1234 now"));
console.log("Ordered 12 pens and 150 sheets".match(/\d+/g));

true true
true
[ '12', '150' ]

Anchors: matching the whole string#

^ matches the start and $ the end of the string. Without them, a pattern can match anywhere inside the text, which is a classic validation bug:

const pin = /^\d{6}$/;
console.log(pin.test("500001"));
console.log(pin.test("PIN 500001"));
console.log(/\d{6}/.test("PIN 500001 or more"));

true
false
true

\b matches a word boundary: /\bcat\b/ finds "cat" but not "category".

Flags#

Groups#

Parentheses create capturing groups, which extract parts of a match. Named groups ((?<name>...)) are easier to read:

const date = "2026-10-24";

const [, year, month, day] = date.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(day, month, year);

const { groups } = date.match(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/);
console.log(`${groups.d}/${groups.m}/${groups.y}`);

24 10 2026
24/10/2026

(?:...) groups without capturing, and a|b means "a or b": /^(?:jpg|png|webp)$/.

Methods#

const post = "Learning #JavaScript and #CSS today! #webdev";

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

for (const m of post.matchAll(/#(\w+)/g)) {
  console.log(`tag "${m[1]}" at index ${m.index}`);
}

console.log("2026-10-24".replace(/(\d+)-(\d+)-(\d+)/, "$3.$2.$1"));
console.log("a, b;c   d".split(/[,;\s]+/));
console.log("hello world".replace(/\b\w/g, (letter) => letter.toUpperCase()));

[ '#JavaScript', '#CSS', '#webdev' ]
tag "JavaScript" at index 9
tag "CSS" at index 25
tag "webdev" at index 37
24.10.2026
[ 'a', 'b', 'c', 'd' ]
Hello World

In a replacement string, $1, $2 (or $<name>) insert captured groups. A replacement function receives each match and returns the new text.

Lookahead#

A lookahead (?=...) checks that something follows, without including it in the match. Several lookaheads at the start can check independent rules, as in this password check:

const strong = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/;

for (const pwd of ["password", "Password1", "Pa1"]) {
  console.log(pwd, strong.test(pwd));
}

password false
Password1 true
Pa1 false

For user-facing validation, separate simple checks with clear messages are usually friendlier than one large pattern (Handling Forms).

Practical patterns#

const patterns = {
  indianMobile: /^[6-9]\d{9}$/,
  pinCode: /^\d{6}$/,
  hexColour: /^#(?:[0-9a-f]{3}){1,2}$/i,
  simpleEmail: /^[^\s@]+@[^\s@]+\.[^\s@]+$/,
};

console.log(patterns.indianMobile.test("9876543210"), patterns.indianMobile.test("1234567890"));
console.log(patterns.hexColour.test("#2F5FE0"), patterns.hexColour.test("#12345"));
console.log(patterns.simpleEmail.test("asha@example.com"), patterns.simpleEmail.test("asha@"));

true false
true false
true false

No regex can fully validate an email address; a simple shape check plus a confirmation email is the practical approach.

Tips#

• Build and test patterns on a site such as regex101.com, which explains every part.
• Add comments or name your patterns: const PIN_CODE = /^\d{6}$/.
• Beware of a regex with the g flag reused with test(): it remembers its lastIndex between calls, which can give alternating results. Create the regex inside the function, or avoid g with test.

Practice#

const text = "Contact: asha@example.com, ravi.k@mail.co.in or call 98765 43210";

const emails = text.match(/[\w.]+@[\w.]+\w/g);
const phone = text.match(/\d{5}\s?\d{5}/)[0].replace(/\s/g, "");

console.log(emails);
console.log(phone);

[ 'asha@example.com', 'ravi.k@mail.co.in' ]
9876543210

Key takeaway#