正则表达式——肯定环视模拟固化分组

$(function () {
var str= '12.123465';
var str1= '12.12';
var str2= '12.1';
var str3= '12.120';
var str4= '12.12300000123';
var str5= '12.123';

alert(str5);
//var x = /^(\d*\.\d\d[1-9]?)\d*$/;
var x = /^(\d*\.\d\d(?=([1-9]?))\2)\d+$/;
if(x.test(str5)){
alert('ok');
}
var str5= str5.replace(x,'$1');
alert(str5);
});