Using RegEx to specify what NOT to match (ASP.NET / VB)

Instead of using regex to remove matched patterns, I'd prefer to use it to remove unmatched patterns. Can this be done?  I have a few scenarios where I want to exclude characters where its easier to define what not to match than than the other way round.

Example:
  strTest="This sentance con^&*tains 3 characters that I would like to replace"
  objRegEx = New RegEx("[a-zA-Z0-9 ]")
  Return UCase(objRegEx.Replace( strTest, "" ))

This will return "^&*"
What I want to return is "This sentance contains 3 characters that I would like to replace"

I'd want to use regex to replace all characters that are NOT (alpha characters, numeric characters OR spaces).

I need a specific solution to the above problem but could also do with a generic way of specifying what patterns are NOT to be matched.