C#: Regex TRIM on the fly?

Regex.Replace(strLog, @"\((\d{1,6})\)", "$1");

I'm not quite sure how to cleanly implement your code with my existing code as follows:

string pattern4 = @"(\((\d{1,6})\))";
Regex r4 = new Regex(pattern4, RegexOptions.Compiled);
Match Port = r4.Match(str);

Actually, your regex has already done that for you - you have groups defined in your regex expression.
Try:

   string port = r4.Match.Groups[r4.Groups.Count-1]; // add .Value to the end for completeness ...
   string port = r4.Match.Groups[r4.Groups.Count-1].Value;

Also, your regex is overkill, you don't need the forst parens.  This just makes the entire match another group, which is redundant.  You will get the same result with:

    string pattern4 = @"\((\d{1,6})\)";

But without a redundant group.

Oh yeah, as drichards said you don't need the outside brackets... so:

  string pattern4 = @"(?<=\()\d{1,6}(?=\))";

Would work just fine.