ASP.Net/C# - Regex Expression

var rx = new Regex(@"[^_]+$");
var tests = new string[]
 {
  "bym338x2101_aev_bk/lasa/a/230862_15",
  "bym338x2101_aev_bk/lasa/a/230862_1",
  "bym338x2101_aev_bk/lasa/a/230862_103"
 };

foreach (var test in tests)
{
 Console.WriteLine( rx.Match(test).Value );
}

/* Output

15
1
103

*/