you don't need to use regular expressions, you can just use String's TrimEnd method, but if you insist,
string xx = "T:,0,e:,n,l:,1500,,";
xx = xx.TrimEnd(',');
Console.WriteLine(xx); xx = "T:,0,e:,n,l:,1500,,";
xx = System.Text.RegularExpressions.Regex.Replace(xx,",+$","");
Console.WriteLine(xx);
string xx = "T:,0,e:,n,l:,1500,,";
xx = xx.TrimEnd(',');
Console.WriteLine(xx); xx = "T:,0,e:,n,l:,1500,,";
xx = System.Text.RegularExpressions.Regex.Replace(xx,",+$","");
Console.WriteLine(xx);
哈哈,好玩儿.