javascript - Regular expression /(\d)(?=(\d{3})+\.)/g

How to add a comma to every three digits of the number to the left of a floating point number, such as converting 12000000.11 into "12,000,000.11"?

function commafy(num){

  return num && num
      .toString()
      .replace(/(\d)(?=(\d{3})+\.)/g, function(, ){
          return  + ',';
      });

}
I don’t know how to understand this regular rule. /(d)(?=(d{3}) .)/g
Not sure how it works