How to Create Regex in JavaScript: Syntax and Examples

In JavaScript, you create a regular expression using either the /pattern/flags literal syntax or the RegExp constructor. Both ways let you define patterns to search or match text.

Syntax

There are two ways to create a regular expression in JavaScript:

• Literal syntax: Use slashes /pattern/flags where pattern is the regex and flags are optional modifiers.
• Constructor syntax: Use new RegExp('pattern', 'flags') where both pattern and flags are strings.

Flags like g (global), i (ignore case), and m (multiline) change how the regex works.

javascript

// Literal syntax
const regex1 = /hello/i;

// Constructor syntax
const regex2 = new RegExp('hello', 'i');

Example

This example shows how to create a regex to find the word "hello" ignoring case, then test if it matches a string.

javascript

const regex = /hello/i;
const text = 'Hello world!';
const result = regex.test(text);
console.log(result);

Output

true

Common Pitfalls

Common mistakes include:

• Forgetting to escape special characters like . or ? in the pattern.
• Using the constructor syntax without double escaping backslashes (e.g., \\d instead of \d).
• Confusing flags or forgetting them when needed.

javascript

// Wrong: missing escape for dot
const wrong = /www.example.com/;
console.log(wrong.test('www.example.com')); // true because dot means any char

// Right: escape dot
const right = /www\.example\.com/;
console.log(right.test('www.example.com')); // true

// Constructor needs double escape
const regex = new RegExp('\\d+', 'g');
console.log('123abc'.match(regex)); // ['123']

Output

true
true
[ '123' ]

Quick Reference

Syntax	Description
/pattern/flags	Regex literal with optional flags
new RegExp('pattern', 'flags')	Regex constructor with pattern and flags as strings
g	Global search (find all matches)
i	Ignore case
m	Multiline mode
\d	Matches any digit (0-9)
\w	Matches any word character (letters, digits, underscore)
.	Matches any single character except newline

Key Takeaways

Create regex in JavaScript using literal syntax /pattern/flags or RegExp constructor.

Use flags like g, i, and m to modify regex behavior.

Escape special characters properly to avoid unexpected matches.

Constructor syntax requires double escaping backslashes.

Test regex with methods like test() or match() to check matches.