Regular Expressions in JavaScript with Examples for Beginners

Introduction to JavaScript Regular Expressions

Regular expressions (regex or regexp) are a powerful tool for searching, replacing, and validating strings based on a given pattern. In JavaScript (and Node.js), they are implemented through the built-in RegExp object.

For a beginner, regex syntax may seem intimidating, but it is actually just a language for describing patterns. By mastering the basic constructs, you will be able to:

• Validate email, phone, URL
• Extract data from text (parsing)
• Replace substrings using complex rules
• Filter and process user input

In this article, we will cover regular expressions in JavaScript with examples on Node.js. All examples can be run in the console or in a runtime environment.

Creating a Regular Expression in JS

In JavaScript, there are two ways to create a regex:

1. Literal syntax (recommended)

const pattern = /hello/;  // Searches for the word "hello"

The pattern is enclosed between two slashes. Flags can be specified after the closing slash (e.g., i — case-insensitive search).

2. RegExp Constructor

const pattern = new RegExp('hello');  // Equivalent to /hello/

Used when the pattern is dynamic (constructed from a variable).

Regular Expression Flags

• i — ignore case (/hello/i will find "Hello", "HELLO")
• g — global search (finds all matches, not just the first)
• m — multiline mode (changes behavior of anchors ^ and $)
• s — dot matches newline character
• u — Unicode support
• y — sticky search (searches only from the lastIndex position)

const regex = /javascript/gi;  // Searches for "javascript" globally and case-insensitively

Key Methods for Working with regex in JavaScript

Let's look at the key methods used with regular expressions.

test() — Checking for a pattern

Returns true or false. The fastest method for validation.

const emailPattern = /^[a-z0-9._%+-]+@[a-z0-9.-]+\\.[a-z]{2,}$/i;console.log(emailPattern.test('user@example.com')); // trueconsole.log(emailPattern.test('invalid-email'));    // false

exec() — Search with details

Returns an array with the match and groups. With the g flag, it remembers the position.

const str = 'Price: 100 rub, discount: 20 rub';const pricePattern = /(\\d+)\\srub/g;let match;while ((match = pricePattern.exec(str)) !== null) {  console.log(`Found: ${match[0]}, number: ${match[1]}`);}// Output:// Found: 100 rub, number: 100// Found: 20 rub, number: 20

match() — Search in a string

A string method. Without the g flag, it returns the first match (like exec). With the g flag, it returns an array of all matches.

const text = 'JS is JavaScript, not to be confused with Java.';const jsMatches = text.match(/Java\\w*/g);console.log(jsMatches); // ['JavaScript', 'Java']

replace() — Pattern-based replacement

Replaces matches with a new string. Supports special characters ($1, $&).

const phone = '+7 (123) 456-78-90';const cleanPhone = phone.replace(/[\\s\\(\\)\\-]/g, '');console.log(cleanPhone); // +71234567890

// Replacement using groupsconst date = '2024-03-15';const reversed = date.replace(/(\\d{4})-(\\d{2})-(\\d{2})/, '$3.$2.$1');console.log(reversed); // 15.03.2024

search() — Index of the first match

const index = 'Hello world'.search(/world/);console.log(index); // 6