Notepad++ RegEx Search/Replace: How to append and prepend a character at start and end of each file line?

Just search for

(.*)

and replace with

"\1"

with regular expression option activated. Regular expressions are working only on a row bases, so (.*) matches the complete row and because of the brackets around you can access the match using \1.

Try searching ^(.*)$ and replacing by "$1".

bye ;)