stringObj.replace(rgExp, replaceText)？replaceText是函数的时候不太明白

function f2c(s) {
  var test = /(\d+(\.\d*)?)F\b/g;// 初始化模式。
  return(s.replace(test,
   function($0,$1,$2) {
return(((($1-32) * 5/9).toFixed(0)) + "C");//
  }
                   )
        );
}
//$0 is32.5F $1 is 32.5 $2 is .5
document.write(f2c("Water freezes at 32.5F and boils at 212F."));
$0,$1,$2这三个参数是怎么传的？