How to Use Regex in JavaScript: Syntax and Examples

In JavaScript, you use RegExp objects or regex literals like /pattern/flags to create regular expressions. You can test patterns with test(), find matches with match(), or replace text using replace() methods.

Syntax

A regular expression in JavaScript can be created using a regex literal or the RegExp constructor.

• Regex literal: /pattern/flags where pattern is the search pattern and flags modify the search behavior.
• RegExp constructor: new RegExp('pattern', 'flags') useful when pattern is dynamic.
• Common flags: g for global search, i for case-insensitive, m for multiline.

javascript

/abc/i;

const regex = new RegExp('abc', 'i');

Example

This example shows how to test if a string contains the word 'hello' ignoring case, and how to replace it with 'hi'.

javascript

const regex = /hello/i;
const text = 'Hello world!';

// Test if 'hello' exists
const hasHello = regex.test(text);
console.log(hasHello); // true

// Replace 'hello' with 'hi'
const newText = text.replace(regex, 'hi');
console.log(newText); // 'hi world!'

Output

true
hi world!

Common Pitfalls

One common mistake is forgetting the global flag g when you want to replace all matches, not just the first. Another is not escaping special characters like . or ? when you want to match them literally.

Also, using the test() method on a regex with the g flag can give unexpected results because it remembers the last index.

javascript

const regexWrong = /hello/;
const text = 'hello hello';

// Only replaces first 'hello'
console.log(text.replace(regexWrong, 'hi')); // 'hi hello'

const regexRight = /hello/g;
// Replaces all 'hello'
console.log(text.replace(regexRight, 'hi')); // 'hi hi'

const regexTest = /hello/g;
console.log(regexTest.test(text)); // true
console.log(regexTest.test(text)); // false (because lastIndex moved)

// Fix: create new regex or reset lastIndex
regexTest.lastIndex = 0;
console.log(regexTest.test(text)); // true

Output

hi hello
hi hi
true
false
true

Quick Reference

Feature	Description	Example
Regex literal	Create regex with slashes	/abc/i
RegExp constructor	Create regex dynamically	new RegExp('abc', 'i')
Flags	Modify search behavior	g (global), i (ignore case), m (multiline)
test()	Check if pattern exists	/abc/.test('abc') // true
match()	Get matches from string	'abc123'.match(/\d+/) // ['123']
replace()	Replace matched text	'abc'.replace(/a/, 'x') // 'xbc'

Key Takeaways

Use regex literals or RegExp constructor to create patterns in JavaScript.

Remember to use flags like 'g' for global and 'i' for case-insensitive searches.

Use test(), match(), and replace() methods to work with regex on strings.

Escape special characters when you want to match them literally.

Be cautious with the 'g' flag and test() method as it maintains state between calls.