Regex Variable in JavaScript

A regular expression, also known as regex, is a sequence of characters that defines a pattern of text to be matched. A regular expression is an object used only with strings.

To learn more about the regular expressions and the various character classes and flags available, you can check the official documentation. This article shortly discusses the regular expression in JavaScript.

Use a Variable in a Regular Expression Using Template Literals in JavaScript

There are two ways to define a regular expression in JavaScript, one by using the regular expression literal where patterns are enclosed inside slashes / and the other by calling the constructor function of the RegExp object. This constructor function takes two parameters, the first is the pattern and the second is the flag.

For this tutorial, we will use the constructor function of the RegExp object and will see how to use a variable within a regular expression in JavaScript. In the below example, we have taken an inputString that contacts various characters and numbers.

Our aim here is to create a pattern, pass it to the RegExp constructor function, and replace all the digits within the string with a word.

let inputString = "I work at 1. Android is developed by 2.";

Since the RegExp constructor function takes the pattern as the first parameter, let’s create a new variable called pattern. We will pass the \\d that will search for the digits inside the string inside this variable.

We will enclose the pattern variable with the \\b character and pass it as the first argument to the RegExp constructor function. But just passing the pattern variable as \\bpattern\\b will not work, throwing an invalid escape sequence error.

We have to use the template literals or template strings for this to work. We will enclose the entire string inside the backticks characters, and then wherever we want to add the pattern variable, we will use the ${} and pass the pattern variable inside the curly braces as shown in the below code snippet.

Code Snippet - JavaScript:

let pattern = "\\d";
let re = new RegExp(`\\b${pattern}\\b`, 'gi');
console.log(inputString.replace(re, "Google"));

And then, we will pass gi as the second argument to the RegExp constructor function. The g will perform a global search on the entire input string, and i will replace all the instances of a specific word provided inside the replace function.

Then we will store the result of the RegExp constructor function inside the re (regular expression) variable. Finally, we will use the string replace() function on the inputString and pass the regex as the first parameter and the new word replaced with the matched.

Output:

I work at Google. Android is developed by Google.

In this case, we want to replace the numbers 1 and 2 with the word Google. So, the regular expression stored inside the re variable will find these two digits within the inputString, and then it will replace these numbers with the word Google with the help of the replace() function.