using System;
using System.Text;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
    	var s = "abbdabab";
    	var idx = 1; // First = 1, Second = 2
    	var search = "ab";
    	var repl = "x";
    	var pat = new Regex($@"(?s)((?:{search}.*?){{{idx-1}}}.*?){search}"); // ((?:ab.*?){0}.*?)ab
		Console.WriteLine(pat.Replace(s, $"${{1}}{repl}", 1));
	}
}
 

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