How to use Variable in a Regular Expression in JavaScript

In this tutorial, we are going to learn about using a variable in a regular expression (regex).

Consider we have a string and we need to remove all yes in that string.

We can do that by using a replace() method.

Example:

const str = "Hello yes what are you yes doing yes"
            // str.replace(regex-pattern,replacevalue)
const newstr = str.replace(/yes/g,'');

console.log(newstr);
//"Hello  what are you  doing "

In the above code, we have passed the regex pattern as an argument to the replace() method instead of that we can store the regex pattern in a variable and pass it to the replace method.

Note: Regex can be created in two ways first one is regex literal and the second one is regex constructor method (new RegExp()).

If we try to pass a variable to the regex literal pattern it won’t work.

const str = "Hello yes what are you yes doing yes"
const removeStr = "yes"
const regex =  `/${removeStr}/g` // wrong way
const newstr = str.replace(regex,''); // won't work

The right way of doing it is by using a regular expression constructor new RegExp().

const str = "Hello yes what are you yes doing yes"
const removeStr = "yes" //variable
const regex =  new RegExp(removeStr,'g'); // correct way
const newstr = str.replace(regex,''); // it works

In the above code, we have passed the removeStr variable as an argument to the new RegExp() constructor method.