I've tried that but as it read from the filenames thus excluded all the cover/comments etc that i would like to keep.
I'm not sure what cover/comments you are interested in. If you are importing with metadata obtained from inside the file, not from metadata constructed from the filename, then regex doesn't apply at all. It only applies to the filename. If you are finding some files as LN, FN and some as FN LN and you are getting your metadata from inside the file, then your problem is that your files have bad/inconsistent internal metadata.
Almost everyone has that problem. Your only options are to: 1) import and get the metadata from the filename, or 2) run regex Search and Replace after import to fix up the bad stuff. Most people use regex and filename, then get the cover from online or open the metadata editor and tell it to grab the cover from inside the file.