using System;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var testInput = "a_b_c_d_e_f";
var items = Regex.Split(testInput, @"(?<!^(?:[^_]*_[^_]*){3})_");
Console.WriteLine(string.Join(" | ", items));
}
}
dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uVGV4dC5SZWd1bGFyRXhwcmVzc2lvbnM7CgpwdWJsaWMgY2xhc3MgVGVzdAp7CglwdWJsaWMgc3RhdGljIHZvaWQgTWFpbigpCgl7CgkJdmFyIHRlc3RJbnB1dCA9ICJhX2JfY19kX2VfZiI7CiAgICAgICAgdmFyIGl0ZW1zID0gUmVnZXguU3BsaXQodGVzdElucHV0LCBAIig/PCFeKD86W15fXSpfW15fXSopezN9KV8iKTsKICAgICAgICBDb25zb2xlLldyaXRlTGluZShzdHJpbmcuSm9pbigiIHwgIiwgaXRlbXMpKTsKCX0KfQ==