RegexFun.cs

public static string Replace(string input, string regex)
{
	var match = System.Text.RegularExpressions.Regex.Match(regex, @"^s(.)(?<Regex>.*?)(?:(?<!\\)\1)(?<Replace>.*?)(?:(?<!\\)\1)$");
	
	var replaced = Regex.Replace(input, match.Groups["Regex"].Value, match.Groups["Replace"].Value);
	
	return replaced;
}

Usage.cs

var replaced1 = Replace("value", "s/value/VALUE/");

Console.WriteLine(replaced1); // VALUE

var replaced2 = Replace("value/value", "s/value\\/value/VALUE/");

Console.WriteLine(replaced2); // VALUE