using (StreamReader stream = new StreamReader(log4NetConfigFilePath))
{
var doc = XDocument.Load(stream);
var start = doc.Element("log4net").Element("mask").Element("start").Value;
var end = doc.Element("log4net").Element("mask").Element("end").Value;
var replace = doc.Element("log4net").Element("mask").Element("replace").Value;
string before = @"/\*.*\*/";
string after = "---";
string test = Regex.Replace("ああ/*testです表示*/", before, after, RegexOptions.Singleline);
System.Console.WriteLine(test);
var xml_before = doc.Element("log4net").Element("mask").Element("before").Value;
var xml_after = doc.Element("log4net").Element("mask").Element("after").Value;
string xml_test = Regex.Replace("ああ/*testです表示*/", before, after, RegexOptions.Singleline);
System.Console.WriteLine(xml_test);
}