Question

I’m parsing a text using C# regex. I want to replace only one specific group in for each match. Here how I’m doing it:

void Replace(){
  string newText = Regex.Replace(File.ReadAllText(sourceFile), myRegex, Matcher, RegexOptions.Singleline);
  //.......   
}

void string Matcher(Match m){
  // how do I replace m.Groups[2] with "replacedText"?
  return ""; // I want to return m.Value with replaced m.Group[2]
}

Answer 1

This should do it:

string Matcher(Match m)
{
    if (m.Groups.Count < 3)
    {
        return m.Value;
    }

    return string.Join("",  m.Groups
                             .OfType<Group>() //for LINQ
                             .Select((g, i) => i == 2 ? "replacedText" : g.Value)
                             .Skip(1) //for Groups[0]
                             .ToArray());
}

Example: http://rextester.com/DLGVPA38953

EDIT: Although the above is the answer to your question as written, you may find zero-width lookarounds simpler for your actual scenario:

Regex.Replace(input, @"(?<=e)l+(?=o)", replacement)

Example: http://rextester.com/SOWWS24307