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Capturing Groups – JavaScript Regex

by | Jul 20, 2023

Regular expressions, commonly known as regex, are a powerful tool in JavaScript for pattern matching and text manipulation. They allow developers to search, extract, and manipulate specific parts of a string efficiently.

One of the most valuable features of regex is capturing groups, which enables us to extract specific portions of matched text.

In this blog post, we will explore the wonders of capturing groups in JavaScript regex and how they can level up your text processing game.

Introduction to Regular Expressions (Regex)

Regular expressions, often abbreviated as regex or regexp, are a powerful tool in programming for pattern matching and text manipulation. They provide a concise and flexible way to describe, search, and manipulate text based on specific patterns.

At its core, a regular expression is a sequence of characters that forms a search pattern. This pattern can include various elements such as literals, character classes, quantifiers, and more, allowing you to create intricate rules for matching strings.

Key Concepts

  1. Patterns: Regular expressions are defined by patterns composed of characters and special symbols. These patterns define the rules for matching text.
  2. Literals: Literal characters in a regex pattern match exactly with the same characters in the target text. For example, the pattern “cat” will match the characters “cat” in a string.
  3. Metacharacters: Special characters in regex, known as metacharacters, carry special meanings. For instance, the dot (.) matches any single character, and the asterisk (*) quantifies the preceding character, allowing for zero or more occurrences.
  4. Character Classes: Character classes, enclosed in square brackets [], define a set of characters. For example, [aeiou] matches any vowel.
  5. Quantifiers: Quantifiers determine the number of occurrences of a character or group. Common quantifiers include *, +, and ?.
  6. Capturing Groups: Parentheses () are used to create capturing groups, allowing you to extract specific portions of the matched text.

Why Use Regular Expressions

Regular expressions are invaluable for tasks such as:

  • Validation: Checking if a string adheres to a specific format, like a valid email address or phone number.
  • Extraction: Capturing specific information from a larger text, such as extracting usernames from an email address.
  • Search and Replace: Performing complex search and replace operations within strings or documents.
  • Parsing: Breaking down complex data structures or documents based on predefined patterns.

In the realm of JavaScript, regular expressions are frequently used with methods like test(), exec(), and replace() to perform various text-related operations.

Understanding regular expressions empowers developers to handle text processing challenges with precision and efficiency. While they might seem daunting at first, mastering regex provides a valuable skill set for anyone working with strings in programming.

Understanding Capturing Groups

A capturing group is a set of parentheses ( ) in a regex pattern that allows us to “capture” the matched text enclosed within those parentheses.

When a regex pattern is matched against a string, any part of the string that matches the content within the capturing group is stored for later use.

Capturing groups are incredibly useful when we need to extract specific information from a complex string or perform replacements selectively.

Basic Syntax of Capturing Groups

To create a capturing group in a regex pattern, simply enclose the desired pattern within parentheses. For example:

const text = "Hello, my email is";
const emailRegex = /(\w+\.?\w+)@(\w+\.\w+)/;

const result = emailRegex.exec(text);

In this example, we have a regex pattern that captures the username and domain of an email address. The matched username will be stored in result[1], while the domain will be stored in result[2].

Accessing Captured Groups

Once we’ve executed a regex pattern against a string using methods like exec() or match(), the captured groups’ content is available in the resulting array.

The entire matched string is stored at index 0, and the captured groups start from index 1 and go upwards. Let’s see an example of accessing captured groups:

const text = "John's phone number is (555) 123-4567";
const phoneRegex = /\((\d{3})\)\s(\d{3})-(\d{4})/;

const result = phoneRegex.exec(text);

const areaCode = result[1];
const firstThreeDigits = result[2];
const lastFourDigits = result[3];

console.log(`Area Code: ${areaCode}`);
console.log(`First Three Digits: ${firstThreeDigits}`);
console.log(`Last Four Digits: ${lastFourDigits}`);


Area Code: 555
First Three Digits: 123
Last Four Digits: 4567

Non-Capturing Groups

In some cases, you might want to use parentheses for grouping in your regex pattern but not capture the matched text. To achieve this, you can use non-capturing groups, denoted by (?: ).

Non-capturing groups are useful when you want to apply quantifiers or alternations to a group without storing the matched content. For example:

const text = "I love cats, but not dogs.";
const petRegex = /(?:cats|dogs)/;

const result = petRegex.exec(text);

console.log(`Found: ${result[0]}`);


Found: cats
<h2>Reusing Capturing Groups</h2>
Captured groups are not only useful for extracting information but can also be used in replacements. When using the <code>replace()</code> method, you can reference captured groups using the <code>$1</code>, <code>$2</code>, etc., notation:

[code lang="js"]
const text = "First name: John, Last name: Doe";
const nameRegex = /First name: (\w+), Last name: (\w+)/;

const replacedText = text.replace(nameRegex, "Last name: $2, First name: $1");



Last name: Doe, First name: John


JavaScript regex capturing groups open up a world of possibilities for manipulating and extracting specific parts of strings.

With the ability to capture text within parentheses and access it later, developers can efficiently process complex data and perform targeted replacements.

By mastering capturing groups, you’ll have a powerful tool at your disposal, enabling you to handle text processing challenges with ease. Happy regexing!


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