Regular Expression for formatting numbers in JavaScript

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This can be done in a single regex, no iteration required. If your browser supports ECMAScript 2018, you could simply use lookaround and just insert commas at the right places:

Search for (?<=\d)(?=(\d\d\d)+(?!\d)) and replace all with ,

In older versions, JavaScript doesn’t support lookbehind, so that doesn’t work. Fortunately, we only need to change a little bit:

Search for (\d)(?=(\d\d\d)+(?!\d)) and replace all with \1,

So, in JavaScript, that would look like:

result = subject.replace(/(\d)(?=(\d\d\d)+(?!\d))/g, "$1,");

Explanation: Assert that from the current position in the string onwards, it is possible to match digits in multiples of three, and that there is a digit left of the current position.

This will also work with decimals (123456.78) as long as there aren’t too many digits “to the right of the dot” (otherwise you get 123,456.789,012).

You can also define it in a Number prototype, as follows:

Number.prototype.format = function(){
   return this.toString().replace(/(\d)(?=(\d{3})+(?!\d))/g, "$1,");
};

And then using it like this:

var num = 1234;
alert(num.format());

Credit: Jeffrey Friedl, Mastering Regular Expressions, 3rd. edition, p. 66-67

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