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; // Counter
var result = string.Empty; // Final result variable
var rx = "[^=]+(?=&)"; // Pattern
for (var m=Regex.Match(s, rx); m.Success; m = m.NextMatch())
{
cnt++;
if (cnt == idx_to_replace) {
result = $"{s.Substring(0, m.Index)}REPLACED{s.Substring(m.Index+m.Length)}";
break;
}
}
Console.WriteLine(result);
}
}
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