Replace [^a-zA-Z0-9] with an empty string.
string str = "This-is-my+++***RegexPattern&Text";
Regex rgx = new Regex("[^a-zA-Z0-9]");
str = rgx.Replace(str, "");
OUTPUT: ThisismyRegexPatternText
If you want to add an exception then you should add this character like this to the regex pattern (let’s assume you wish to exclude the ampersand):
[^a-zA-Z0-9 &]
string str = "This-is-my+++***RegexPattern&Text";
Regex rgx = new Regex("[^a-zA-Z0-9 &]");
str = rgx.Replace(str, "");
OUTPUT: ThisismyRegexPattern&Text