A pure regex way is possible, but is not really very efficient if your pattern is complex.
var s = "=StringOne&=StringTwo&=StringThree&=StringFour&";
var idx = 2; // Replace this occurrence
var result = Regex.Replace(s, $@"^(=(?:[^=&]+&=){{{idx-1}}})[^=&]+", "${1}REPLACED");
Console.WriteLine(result); // => =StringOne&=REPLACED&=StringThree&=StringFour&
See this C# demo and the regex demo.
Regex details
^ - start of string(=(?:[^=&]+&=){1}) - Group 1 capturing:
= - a = symbol(?:[^=&]+&=){1} - 1 occurrence (this number is generated dynamically) of[^=&]+ - 1 or more chars other than = and & (NOTE that in case the string may contain = and &, it is safer to replace it with .*? and pass RegexOptions.Singleline option to the regex compiler)&= - a &= substring.
[^=&]+ - 1 or more chars other than = and &
The ${1} in the replacement pattern inserts the contents of Group 1 back into the resulting string.
As an alternative, I can suggest introducing a counter and increment is upon each match, and only replace the one when the counter is equal to the match occurrence you specify.
Use
var s = "=StringOne&=StringTwo&=StringThree&=StringFour&";
var idx_to_replace = 2; // Replace this occurrence
var cnt = 0; // Counter
var result = Regex.Replace(s, "[^=]+(?=&)", m => { // Match evaluator
cnt++; return cnt == idx_to_replace ? "REPLACED" : m.Value; });
Console.WriteLine(result);
// => =StringOne&=REPLACED&=StringThree&=StringFour&
See the C# demo.
The cnt is incremented inside the match evaluator inside Regex.Replace and m is assigned the current Match object. When cnt is equal to idx_to_replace the replacement occurs, else, the whole match is pasted back (with m.Value).
Another approach is to iterate through the matches, and once the Nth match is found, replace it by splitting the string into parts before the match and after the match breaking out of the loop once the replacement is done:
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); // => =StringOne&=REPLACED&=StringThree&=StringFour&
See another C# demo.
This might be quicker since the engine does not have to find all matches.
