正则表达式：为什么匹配的是ab而不是a啊

Question

var s = 'Hi, do you (know) your abcs?';
const regex1 = /ab?/gi;
console.log(regex1.exec(s));

Answer 1

var s = 'Hi, do you (know) your abcs?';
const regex1 = /ab??/gi;
console.log(regex1.exec(s));

Answer 2

是ab而不是abc？因为理应是ab……

'...abc.'.match(/ab.?/gi)才为abc

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补充：/ab??/非贪婪模式下结果为"a"