If this is your site, my feedback is, to have a more clear cut destinction between the explaination and the task at hand.
I'd add some more feedback:
* Add some detail about which regex flavor is used for the lessons, if they are meant to be common features between the flavors listed under 'References & More', again, mention it somewhere. For example, someone trying to learn regex for grep/sed/awk will discover that `\d` will not work.
* I get that the lessons are not meant to cover everything, for example lookarounds/non-greedy/backreferences aren't covered, but I'd feel word boundaries after the line anchors would make a good addition. Edit Some of these are covered in lesson 15, but that is not linked in the sidebar under 'Lesson Notes'