Regular Expressions (RegEx) in JavaScript

A regular expression is a syntax that represents a search pattern in a string of characters.

In JavaScript, regular expressions are used to perform string matching and manipulation operations. For example,

• Search and replace parts of a string
• Validate data formats (e.g., email addresses, phone numbers)
• Split strings based on specific patterns

Regular expressions may seem complicated at first due to their compact syntax, but they offer great flexibility and power once mastered.

Regular Expressions Course

Learn to program in the introduction to programming course

Creating Regular Expressions in JavaScript

In JavaScript, regular expressions can be defined in two ways: using a regular expression literal or the RegExp constructor.

Regular Expression Literal

The syntax for creating a regular expression using a literal is:

const regex = /expression/flags;

For example,

const regex = /abc/;

In this case,

• /abc/ is a regular expression that matches the string “abc”.

RegExp Constructor

The syntax for creating a regular expression using the RegExp constructor is:

const regex = new RegExp(expression, 'flags');

For example, this creates a regular expression equivalent to /abc/

const regex = new RegExp('abc');

Methods for Working with Regular Expressions

JavaScript provides several useful methods for working with regular expressions through the RegExp property and string (String) methods.

RegExp.test()

The test() method of RegExp is used to check if a regular expression matches a string.

const regex = /hola/;

console.log(regex.test('hola mundo')); // true
console.log(regex.test('adiós mundo')); // false

RegExp.exec()

The exec() method of RegExp searches for a match in a string and returns an array with the match or null if there is no match.

const regex = /(\d+)/;
const result = regex.exec('Tengo 2 manzanas');

console.log(result); // ["2", "2"]

String.match()

The match() method of String is used to get the matches of a regular expression in a string. It returns an array with the matches or null if there is no match.

const text = 'La fecha es 2024-08-14';
const regex = /\d{4}-\d{2}-\d{2}/;

const result = text.match(regex);

console.log(result); // ["2024-08-14"]

String.replace()

The replace() method of String is used to replace parts of a string that match a regular expression.

const text = 'Mi número de teléfono es 123-456-7890';
const regex = /\d{3}-\d{3}-\d{4}/;

const newText = text.replace(regex, 'XXX-XXX-XXXX');

console.log(newText); // "Mi número de teléfono es XXX-XXX-XXXX"

String.split()

The split() method of String is used to split a string into an array using a regular expression as a delimiter.

const text = 'uno,dos,tres';
const regex = /,/;

const result = text.split(regex);

console.log(result); // ["uno", "dos", "tres"]