string aa = "大发放阿发13912731231大大法国客户131大幅13131阿发"; aa = Regex.Replace(aa, @"(\d{4})(\d+)", m => m.Result("$1") + new string('*', m.Result("$2").Length));
string aa = "大发放阿发13912731231大大法国客户131大幅13131阿发"; aa = Regex.Replace(aa, @"(?<=[^\d]+\d{4})(\d+)(?=[^\d])", m => new string('*', m.Result("$1").Length)); Console.WriteLine(aa);
aa = Regex.Replace(aa, @"(\d{4})(\d+)", m => m.Result("$1") + new string('*', m.Result("$2").Length));
aa = Regex.Replace(aa, @"(?<=[^\d]+\d{4})(\d+)(?=[^\d])", m => new string('*', m.Result("$1").Length));
Console.WriteLine(aa);