RegExp syntax

Learn to create patterns with the RegExp literal and object.

What is a RegExp?

A RegExp (regular expression) searches for a pattern inside text. JavaScript gives you two ways to create one: a literal and a constructor.

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

Both represent the same pattern. The literal is short and readable; the constructor is handy when you build a pattern at runtime from a string, for example from a user's search term.

Flags

Add a flag at the end of a pattern to change how it behaves. The three most common flags are:

i — case-insensitive matching
g — find all matches (global)
m — multiline mode

/cat/i.test("Cat"); // true

With the constructor the flag is the second argument: new RegExp("cat", "i").

Exercise Test a cat pattern

Assign a regex literal matching "cat" to catPattern. Then test it on the string "concatenate" with .test() and log the result with console.log.

Show example solution

/cat/. The test method returns true/false.
Example solution:
let catPattern = /cat/;
console.log(catPattern.test("concatenate"));

Test yourself

Which is a valid RegExp literal?
- "/cat/"
- /cat/
- RegExp("cat")
- #cat#
Show answer

/cat/ — Correct! /cat/ is a regex literal; it goes between two slashes.

Make a dog pattern literal

const dogPattern = ____;
console.log(dogPattern.test("hotdog"));

Show answer

const dogPattern = /dog/;
console.log(dogPattern.test("hotdog"));

Match the creation forms

/cat/

literal
new RegExp('cat')

constructor
/cat/i

case-insensitive
/cat/g

global
Which flag makes matching case-insensitive?
- i
- g
- m
- s
Show answer

i — Correct! The i flag turns off case sensitivity.

Next lesson: Character classes

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies