How to use a Variable in a Regular Expression in JavaScript

Regular expressions are useful because they are a powerful way to match patterns in strings.

However, sometimes the thing you want to match for is coming from a variable. For example, you might want to match for a string that is stored in a variable.

In this post, we will learn how to use a variable in a regular expression in JavaScript.

Using a variable in a regular expression

Let's say that you are trying to replace the word "dog" with the word "cat" in a string. You could do this with the following code:

JAVASCRIPT
const string = "The dog is happy.";

const newString = string.replace("dog", "cat");

console.log(newString);

BASH
The cat is happy.

Now, this works but it isn't using a regular expression. Let's accomplish the same task as above but using a regular expression.

JAVASCRIPT
const string = "The dog is happy.";

const newString = string.replace(/dog/, "cat");

console.log(newString);

BASH
The cat is happy.

Now, let's say instead of replacing the word "dog" with the word "cat", you want to replace the word that is stored in a variable.

We can't just use the variable in the regular expression because it will be interpreted as a string. Instead, we need to use the RegExp constructor.

JAVASCRIPT
const string = "The dog is happy.";

const word = "dog";

const regex = new RegExp(word);

const newString = string.replace(regex, "cat");

console.log(newString);

BASH
The cat is happy.

By using the RegExp constructor, we can use a variable in a regular expression.

Conclusion

In this post, we learned how to use a variable in a regular expression in JavaScript.

Simply use the RegExp constructor to create a regular expression from a variable.

Thanks for reading!

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