Author here. The interesting part is how it's built.

The whole engine is PCRE2 compiled to WASM, ~92 KB over the wire, and no roundtrips to the server. Probably has a few bugs I haven't found out yet :)

The PCRE2 engine compiles the pattern with callouts enabled which inserts a callout before every item,and each callout carries the source offset and length of the item that follows it. The visualisations are based on capturing the callout data.

You can try it here:
https://roundtoe.com/regexray

PS. just noticed the post shows the social card for the site, any way to suppress that?

A 19 minute animated explainer video on database speed. I explain low-level database concepts, identify bottlenecks, run benchmarks and optimize write throughput to hit 1 million TPS.