using System;

using System.Linq;

​

var files = new string[]{

"123456_SCARBROUGH_MEDLIN___ASSOCIATES_INC_LiveDebug_d50d0b5a-4ea2-4422-be9e-00b4927eab8d_11082023144005_1_mail.pdf",

"123456_ERNIE__LOPEZ_Staging_64567d68-c3a0-4fa8-b3fb-2d3e993325d2_11082023123321_1_mail.pdf",

"3220648_FIVE_RIVERS_TRUCKING_LLC_qa_3eccae4f-2f36-4e15-967b-d41b63a8bd26_11082023095001_1_mail.pdf",

};

​

foreach (var environment in files.Select(f => f.Split("_")[^5]))

    Console.WriteLine(environment);

​