Regular Expression for formatting numbers in JavaScript

Question

I need to display a formatted number on a web page using JavaScript. I want to format it so that there are commas in the right places. How would I do this with a regular exp

Answer 1

If you really want a regex, you can use two in a while loop:

while(num.match(/\d{4}/)) {
    num = num.replace(/(\d{3})(,\d|$)/, ',$1$2');
}

And if you want to be fancy, you can format numbers with decimal points too:

while(num.match(/\d{4}(\,|\.)/)) {
    num = num.replace(/(\d{3})(,\d|$|\.)/, ',$1$2');
}

Edit:

You can also do this with 2 regular expressions and no loop, splits, joins, etc:

num = num.replace(/(\d{1,2}?)((\d{3})+)$/, "$1,$2");
num = num.replace(/(\d{3})(?=\d)/g, "$1,");

The first regex puts a comma after the first 1 or 2 digits if the remaining number of digits is divisible by three. The second regex places a comma after every remaining group of 3 digits.

These won't work with decimals, but they work great for positive and negative integers.

Test output:

45
3,856
398,868,483,992

635
12,358,717,859,918,856
-1,388,488,184