CodePen Embed - How does JavaScript display a numeric thousand position using a regular expression?

n = "1279834847944074100465236.33";
re = /(\d{1,3})(?=(\d{3})+(?:$|\D))/g ;
n1 = n.replace(re,"$1,");

document.write("Before:" + n + "<br/>After:" + n1);

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