JavaScript Regular Expressions

A regular expression (regex) is a pattern to find or check text. In JavaScript it is written between slashes: /pattern/flags.

const pattern = /cat/;
console.log(pattern.test("The cat sat"));
console.log(pattern.test("The dog sat"));

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Flags

Flag	Meaning
i	Ignore upper and lower case
g	Find all matches, not only the first
m	Multi-line

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Pattern building blocks

Pattern	Meaning
.	Any character
\d, \w, \s	A digit, a word character, a space
[abc]	One of a, b or c
[0-9]	A digit
^, $	Start and end of the text
*, +, ?	0 or more, 1 or more, 0 or 1
{3}, {2,4}	Exactly 3, between 2 and 4
( )	A group
|	Or

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test, match and replace

const text = "Call 98765 or 12345 today";

console.log(/\d{5}/.test(text));
console.log(text.match(/\d{5}/g));
console.log(text.replace(/\d{5}/g, "#####"));
console.log("Hello".match(/l+/)[0]);

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Check a format

const phone = /^[6-9]\d{9}$/;          // 10 digit mobile number
console.log(phone.test("9876543210"));
console.log(phone.test("12345"));

const email = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
console.log(email.test("robin@example.com"));

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Groups

const date = "2025-01-15";
const [, year, month, day] = date.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(year, month, day);

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Summary

• Patterns go between slashes, with flags like g and i.
• test() checks, match() finds, replace() changes.
• Anchors ^ and $ check the whole text.
• Related: Forms and Validation.

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