using System;

using System.Text;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string txt = @"

Full Environment stacktrace:

   a következő helyen: System.Environment.GetStackTrace(Exception e, Boolean needFileInfo).. .gz.base64:

H4sICF8YNFgCAGpvenNpAJ2PMU7DUBBEeyTusKWDIhfQ0QUrhaUothJDG/3YQ2L8vd9ary05t+Ay

NBQUiPNwBb6QYlFECkq32tmdN/P9/kFEhqrPtx5a4fD1SnvYAXxP66FV1OGc+1Ic12ANd9DNWk1e

ZWJyBBO6vjr3voSGC3MYZu3A+QptZzWMXN1YKIKYNVWhroVkrgJPLvNLxSlyRRFz720iY+3WhwyS

7Ytfk/xeTelSWulHhvwlxsmxQxGc0MmMs+c+evDdLUHESeQKjBvu6odB0f4jSLYXmKLkXbgZ2aXj

Y9UwhTw7qU9pwVn+lJZGyx5JD7GmaVDcUJM8zVeLWeqz/QDz1qqPJQIAAA==

<hereinafter strace#xtDcC6>

";

        StringBuilder b64 = new StringBuilder(), result = new StringBuilder();

        var re = new Regex(@"^\s*H4sI[A-Za-z0-9+/]{20,}={0,3}\s*$");

        foreach (string line in txt.Split('\n'))

        {

            if (!re.IsMatch(line)) {

               if (b64.Length == 0) result.AppendLine(line.TrimEnd('\r'));

               continue;

            }

            if (b64.Length == 0)

                re = new Regex(@"^\s*[A-Za-z0-9+/]+={0,3}\s*$");

            b64.Append(line.Trim());

            if (line.Contains("="))

                break;

        }

        result.Replace(".. .gz.base64:", "");

        if ((b64.Length & 3) != 0)

            b64.Append('=', -b64.Length & 3);

        byte[] b = Convert.FromBase64String(b64.ToString());

        if (4 < b.Length && b[0] == 31 && b[1] == 139 && b[2] == 8 

           && (b[3] == 0 || b[3] == 8)) {  // the base64 content IS a Gzip

           using (var sr = new System.IO.StreamReader(

             new System.IO.Compression.GZipStream(new System.IO.MemoryStream(b),

             System.IO.Compression.CompressionMode.Decompress)))

             result.Append(sr.ReadToEnd());

        } else {  // the base64 content is NOT a Gzip

           result.Append(Encoding.UTF8.GetString(b));

        }

        Console.WriteLine(result.ToString());

​

    }

}