Regular Expression to insert using replace method in Javascript

Regular Expression to insert using replace method in Javascript

Math.round(num1/num2).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",")
I have this and if I put
var num1 = 123456789;
var num2 = 10000;
I get 12,346 but I do not understand how it works using replace method.
Doesn't replace() method replace something, which means a character has to
be deleted? But how come this function can insert comma between
characters? I understand the regex one by one but do not understand how it
works in total. Thanks!