简单的正则表达？解决方案

简单的正则表达？
201107131222475391201107131225438372201107180938036337

正则怎样表达每18位加一个逗号？
201107131222475391，201107131225438372，201107180938036337 变成这样

[解决办法]
用，\d{18}，匹配出来三条记录，然后再加上，就行了
[解决办法]
"^\d{18}(\,\d{18})*$"
[解决办法]

C# code

//EASYvoid Main(){    string str="201107131222475391201107131225438372201107180938036337";    str=Regex.Replace(str,"(?<=\\G.{18})(?!$)",",");    Console.WriteLine(str);    //201107131222475391,201107131225438372,201107180938036337}