Regular expression: Replace anything that isn't alphanumeric with a space?

Hi there,
I'm writing a function in C# that will clean up a filename that a user might upload.
One of the requirements is to replace any character that isn't alphanumeric with a space, but I can't seem to get the regular expression working to do so?
I've include my code below, thanks for any help :)
// Replace miscellaneous characters with a space inputFilenameRegex = new Regex(@"^[a-zA-Z0-9]"); inputFilename = inputFilenameRegex.Replace(inputFilename, " ");


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