js代码示例：
let str = '13888888888';
let reg = /(?<=\d{3})(\d{4})/;
str.replace(reg,"****"); //138****8888
dart的一种写法
String phoneNumber = '13888888888';
String phoneNumberStr = phoneNumber.replaceFirst(new RegExp(r'\d{4}'), '****', 3);
replaceFirst用法：
String replaceFirst(Pattern from, String to, [int startIndex = 0])
dart:core
Returns a new string in which the first occurrence of [from] in this string is replaced with [to], starting from [startIndex]:
'0.0001'.replaceFirst(new RegExp(r'0'), ''); // '.0001'
'0.0001'.replaceFirst(new RegExp(r'0'), '7', 1); // '0.7001'
