The research of regular expression like the character "\n" does not give any result on a page even if the page contains such characters and the option regular expression is checked.
To reproduce:
1) Go to https://en.wikipedia.org/w/index.php?title=Newline&action=edit
2) Switch to visual mode
4) Check the option Regular expression and search for "\n"
This has been check on English Wikipedia and French Wikisource with Firefox browser under Ubuntu.