How To Test Regex In Javascript

In this blog post, we will learn how to test regex (short for Regular Expressions) in JavaScript. Regular expressions are a powerful tool for performing pattern matching and searching in strings. They can be used for various purposes such as validating user inputs, manipulating strings, and much more.

Creating a Regex

First, we need to create a regex object. In JavaScript, we can create a regex object using the RegExp constructor, or by using regex literals (enclosed by forward slashes).

const regex1 = new RegExp('pattern', 'flags');
const regex2 = /pattern/flags;

The pattern is the actual regex pattern we want to match, and the flags are optional modifiers that affect the regex search (e.g., ‘g’ for global search, ‘i’ for case-insensitive search, and ‘m’ for multiline search).

Testing a Regex

Once we have a regex object, we can use the test() method to test if the regex pattern matches a given string. The test() method returns true if the pattern matches the string, and false otherwise.

const regex = /hello/;
const str1 = 'Hello world';
const str2 = 'Goodbye world';

console.log(regex.test(str1)); // false
console.log(regex.test(str2)); // false

const caseInsensitiveRegex = /hello/i;
console.log(caseInsensitiveRegex.test(str1)); // true

In the example above, the /hello/ regex does not match the string ‘Hello world’ because the regex is case-sensitive by default. To make the regex case-insensitive, we can add the ‘i’ flag, as shown in the /hello/i regex.

Using String Methods

In addition to the test() method, JavaScript also provides string methods that work with regex, such as match(), search(), replace(), and split().

const regex = /\d+/;
const str = 'My phone number is 123-456-7890';

const result = str.match(regex);
console.log(result); // ["123", index: 19, input: "My phone number is 123-456-7890", groups: undefined]

In the example above, the match() method returns an array containing the matched substrings, along with additional information such as the index of the match and the input string. If no match is found, the method returns null.

Online Regex Testers

If you want to quickly test your regex patterns, there are many online regex testers available, such as regex101, regexr, and regexpal. These tools allow you to test your regex patterns against sample texts and provide valuable debugging information.


Regular expressions are a powerful tool for working with text data in JavaScript. By learning how to create and test regex patterns, you can greatly improve your ability to process and manipulate strings in your applications.