(why is this its own instruction instead of a register? i guess having a special register would mean you need to check writes to every register to see if it was the special output register. however, you already have that with pc...)

An instruction felt like the right thing to do, because it's an "action". pc is also its own "action" after all.

In the future I'll probably change the flush instruction significantly. I found it to be a poor fit for the needed flexibility. An instruction that states "highlight range x1 to x2 in color x3" would be slower, but also allow for highlighting regex capture groups.

this feels like a vim-ism; everyone else uses \b for word boundary... right?

\b only matches boundaries between a non-word and word character. This is different from \>, which doesn't care about the preceding character. It simply checks if the next character is a non-word character while not consuming it (look-ahead).

I forgot where I took \> from. It's not unique to Vim I believe, but I'm sure I saw it there as well.

Ah yes, Rust also has it as "\b{end}, \> end-of-word boundary assertion". This is technically a "\b{end-half} half of a end-of-word boundary assertion" but that's too verbose so whatever.