using System;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var line = "Keeping the line intact, replace me, but not you, keep this too.\n\nRetaining me, replace one, but not two, ignore three.";
        var res = Regex.Replace(line, @"replace (?<target>.*), but not (?<source>.*),",
                   m => "replace " + m.Groups["source"].Value.Replace("o", "1") + ", but not " + m.Groups["source"].Value + ",");
        Console.WriteLine(res);
	}
}

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