Thanks for mentioning the tooltip popups. I didn't realize RegExr used mouseover/hover events on the matching text since the other regex sites don't do that. (It seems like non-obvious UX/UI but I now notice that the bottom left corner does say "Roll over a match or expression for details" -- but who reads the instructions?!?! Nobody's got time fo 'dat.)

Nevertheless, I prefer explicit color-coding highlighting the boundaries instead tooltips displaying the offset numbers.