Basically a dup of [this question using php](https://stackoverflow.com/questions/310572/regex-in-php-to-match-that-arent-html-entities), but I need it for C#.

I need to be able to replace any `&` that is not currently not any HTML entity (e.g. `&amp;`) before outputting to screen. I was thinking a regex, but I'm not sure if .Net has something built in that will do this.