using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var s = "=StringOne&=StringTwo&=StringThree&=StringFour&";
var idx_to_replace = 2; // Replace this occurrence
var cnt = 0;
var result = Regex.Replace(s, "[^=]+(?=&)", m => {
cnt++; return cnt == idx_to_replace ? "REPLACED" : m.Value; });
Console.WriteLine(result);
}
}
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