Stripping Out Unwanted Characters From A String

I'm trying to strip out none alphanumeric characters (and a couple of punctuation marks) from a string before inserting into a database.

I've put in my code below, basically I thought I'd loop through each character in turn and pass it through a regular expression to check its validity. If it passes I'll add it to a global variable ('validString') if it fails I'll remove it.

But I can't get my Mid() function to accept 'i' as a value so it won't loop. Code: