全网挑战字符串小于9的字符前加0

 let str = "2021-1-1";
 str.replace(/-(\d{1})/ig,"-0$1").replace(/-0(\d{2})/ig,"-$1") // 2021-01-01

思路1

正则先给字符串加0然后判断三个的值去0

str.replace(/-(\d{1})/ig,"-0$1").replace(/-0(\d{2})/ig,"-$1")

然后得到结果: 2021-01-01

思路2

正则使用边界\b

"2021-1-12".replace(/-(\d{1})\b/ig,"-0$1")

得到结果："2021-01-12"