string text = string.Empty;
using (System.IO.StreamReader file = new System.IO.StreamReader(@"e:\test.txt"))
{
text = file.ReadToEnd();
}
text = System.Text.RegularExpressions.Regex.Replace(text, @"[^a-zA-Z0-9\s]", string.Empty);
System.IO.File.WriteAllText(@"e:\test.txt", text);