为什么下面这个代码不能把id换成class

Question

var tabletd_gg=document.getElementById('gg_blank').innerHTML;    var count = 0;    var res_gg = tabletd_gg.replace(/~/g, function(x){ var symbol = count++ == 1 ? '~<br/>' : x; return symbol;});    document.getElementById('gg_blank').innerHTML=res_gg;效果是这样的转换为什么用getElementsByClassName()就失效了？