Piggybacking onto this to mention my go-to online RegEx editor: RegExr. It lets you test the regex as you type, explains the particular symbols used, as well as has a sidebar where you can see different pattern types categorically. I’ve been using it for almost 2 years now, and haven’t had any reason to use much else (after I discovered this).
Also Joplin! It’s a cross-platform Markdown notes app with many syncing options (including local storage, Dropbox, Nextcloud, and others). I’ve been using it for 2 years now, after trying different apps to see what fits. I’ve been using local storage and Syncthing to keep my Android phone and Linux PC synced, and really like it. It even lets you use an external text editor to edit your notes!