Let’s say we have a variable, text, assigned to a string:

let text = "the cat sat on the mat";

We want to replace the word “the” with the word “no”.

A JavaScript string has a nice replace method that we can use to do this. So, let’s give this a try:

text = text.replace("the", "no");

The text variable now contains the value “no cat sat on the mat”. This perhaps isn’t what we’d hoped for - only the first “the” has been replaced.

The first, parameter in the replace method can actually be a regular expression which we can use to specify that we want to replace all the instances of the matched word:

text = text.replace(/the/g, "no");

The g at the end of the regular expression tells the replace method to match all instances.

So, the text variable now contains the value “no cat sat on no mat”. Great - we managed to replace all the instances of “the” with “no”!

Let’s look at a slightly different example now:

let text = "The cat sat on the mat";

Again, we want to replace all the instances of the word “the” with “no”. However, this time the two instances of “the” are in different cases - “The” and “the”.

So, just doing the following:

text = text.replace(/the/g, "no");

… will give us “The cat sat on no mat”.

Not quite what we wanted!

The solution is simple though:

text = text.replace(/the/gi, "no");

We have extended the regular expression and specified a case insensitive match with i.

So, the text variable now contains the value “no cat sat on no mat”.

Nice!

Below is a working example in CodePen:

Open working example