Walking the filesystem tree and parallelized regex search mostly works. I'm using the PCRE2 library for regexes.
Filtering files by name doesn't work yet at all and I think that will have to be also parallelized.
A rough draft of the UI is also done, just enough to test things. I'll probably have to make my own controls because the textbox in raygui seems to be very basic (i.e. can't select text, no key repeating).