JavaScript RegExp (Regular Expression)

A regular expression is a special sequence of characters that defines a search pattern, typically used for pattern matching within text. It’s often used for tasks such as validating email addresses, phone numbers, or checking if a string contains certain patterns (like dates, specific words, etc.).

In JavaScript, RegExp is an object that is used to create regular expressions, which are patterns used to match character combinations in strings.

// The /i flag makes the pattern case-insensitive
const patt = /Geeks/i; 
const s1 = "geeksforgeeks";
const s2 = "forgeeks";

console.log(patt.test(s1));
console.log(patt.test(s2));

true
true

Creating a RegExp in JavaScript

There are two primary ways to create a RegExp in JavaScript

1. Using the RegExp Constructor

The RegExp constructor is useful when you need to create a regular expression dynamically, such as when the pattern or flags might change based on user input or other conditions.

let pattern = "hello";  // Pattern to match
let flags = "i";  // Case-insensitive flag
let regex = new RegExp(pattern, flags);

let s = "Hello world";

console.log(regex.test(s));

true

• Here, we dynamically create a RegExp object using the RegExp() constructor.
• The pattern “hello” will match the string “Hello” because of the ‘i’ flag, which makes the search case-insensitive.
• regex.test(s) returns true because “Hello” matches the pattern “hello” in a case-insensitive manner.

2. Using Regular Expression Literal

This is the most common and simple way to create a regular expression in JavaScript. It’s especially useful when the pattern is static (doesn’t change dynamically).

let regex = /hello/i;

let s = "Hello world";

console.log(regex.test(s));

true

• In this example, we define the regular expression directly using literal notation.
• /hello/i means we’re looking for the word “hello”, and the ‘i’ flag ensures the search is case-insensitive.
• regex.test(s) returns true because “Hello” matches the pattern “hello” case-insensitively.

Regular Expression Modifiers can be used to perform multiline searches which can also be set to case-insensitive matching: 

Regular Expression Brackets can Find characters in a specified range

Regular Expression Metacharacters are characters with a special meaning:

Regular Expression Quantifiers are used to define quantitiesoccurrence

Regular Expression Object Properties:

Regular Expression Object Methods:

Use Cases of Regular Expressions

1. Validating Email Addresses

Regular expressions are frequently used for validating emails. Here’s an example that matches most email formats

let regex = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6}$/;
console.log(regex.test("user@example.com"));

true

2. Extracting Specific Data from a String

You can use regular expressions to extract specific data from a string, like extracting all numbers from a text.

let regex = /\d+/g;
let res = "There are 123 apples and 456 oranges".match(regex);
console.log(res);

[ '123', '456' ]

3. Replacing Substrings

Regular expressions are often used in replacing certain substrings in text.

let regex = /foo/g;
let res = "foo bar foo".replace(regex, "baz");
console.log(res);

baz bar baz