Assume you want to replace symbols which are not digits or letters (and _ character as @Guffa correctly pointed):
string input = "Hello@Hello&Hello(Hello)";
string result = Regex.Replace(input, @"[^\w\d]", ",");
// Hello,Hello,Hello,Hello,
You can add another symbols which should not be replaced. E.g. if you want white space symbols to stay, then just add \s
to pattern: \[^\w\d\s]