There is something seriously wrong with the search and replace facility in v. 4.2.4.2 In Linux Mint 17 Mate (the latest version). Every time I start a search or a S&R, I have to turn “regular expression” back on, or it doesn’t work. If some people like me want it always on, and others want it always off, why not offer a choice? You should also let people choose whether to leave Other Options on or off. As it stands, one has to hit it every time in order to turn regular expressions on. Should I file a bug report on this?
Advice to upgrade to 4.3.2.2. Since you’re on Linux Mint, use https://launchpad.net/~libreoffice/+archive/ubuntu/ppa.