How do I strip non-alphanumeric characters (including spaces) from a string?

How do I strip non alphanumeric characters from a string and loose spaces in C# with Replace?

I want to keep a-z, A-Z, 0-9 and nothing more (not even " " spaces).

"Hello there(hello#)".Replace(regex-i-want, "");

should give

"Hellotherehello"

I have tried "Hello there(hello#)".Replace(@"[^A-Za-z0-9 ]", ""); but the spaces remain.