Hi Thomas, and another try:In the 'Find' field call the context menu. That is a right click or Context-menu key on the key board. Choose 'Special Character'. The Reverse Solidus is at code point U+5c. That is is group "Basic Latin".
I don't know whether a Japanese LibreOffice has a shortcut for the 'Special Character' dialog. In an English LibreOffice it would be 'Ctrl+Shift+S'.