(this is in linkgrabber but I think the same bug is in download section too?(not 100% sure))
this is a simple example, where the spaces that were _ were not caught (somehow) by the re.replace, but it can get more complicated. sometimes I have to do that right-click rename action multiple times to get all the packages and filenames to finally comply with the regex.
I usually use [^A-Za-z0-9\.-] as I had had the best results with dupe detection with that regular expression.