面试题正则,？和小括号()的用法

面试题正则,？和小括号()的用法

var con= “mom and dad and baby”;
var regP= /mom (and dad (and baby)?)?/ig;
var matches = (con);
// 问题 console.log(matches[2]);输出什么?
console.log(matches[2]);
执行结果：

console.log(matches[2]);//and baby
matches
(3) [“mom and dad and baby”, “and dad and baby”, “and baby”, index: 0, input: “mom and dad and baby”, groups: undefined]0: "mom and dad and baby"1: "and dad and baby"2: "and baby"groups: undefinedindex: 0input: "mom and dad and baby"length: 3__proto__: Array(0)
菜鸟上类似的例子：.html