Definitely does not look like a backtracker to me.
One interesting thing I found is that it appears to support intersection and negation, as documented here: https://github.com/arnoldrobbins/mcilroy-regex/blob/master/R...
Running `make` will build its `grep` utility, and using `grep -A` enables the intersection and negation features.
Yes, it looks kinda interesting. Am still scratching my head at the idea that summarizing how it works isn't anywhere outside the source base.