为什么使用花括号调用不带参数的函数？

Question

我尝试将函数作为替换方法的第二个参数传递，但仅在不使用大括号的情况下起作用？！

   function reg1(match,amount,unit){
     amount=Number(amount)-1;
     if(amount==1){
       unit=unit.slice(0,unit.length-1) 

     }
     else if(amount==0){
       amount="no"
     }
     return amount+" "+unit 
   }

    undefined
    "1 lemon,2 cabbages and 103 eggs".replace(/(\d+) (\w+)/g,reg1())
    "NaN undefined,NaN undefined and NaN undefined"
     "1 lemon,2 cabbages and 103 eggs".replace(/(\d+) (\w+)/g,reg1)
     "no lemon,1 cabbage and 102 eggs"