Nifty demonstration of live coding a grammar with Pharo, and exploring the different possible parse trees for an inputs (e.g. if you change associativity): https://www.youtube.com/watch?v=FFifte3YFUo
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Steve Jobs famously asked for smooth scrolling (i.e. not line-by-line) in a Smalltalk demonstration and they changed the UI live.
Here's a lovely 2 minute YouTube video demonstrating that modification: https://www.youtube.com/watch?v=eEz08IlcNMg
This comparison of OpenSmalltalkVM and TruffleSqueak is a really effective demonstration of JITs.
TruffleSqueak initially has a lower framerate, but eventually exceeds OpenSmalltalkVM. As soon as the GUI is used (at 3:30), the framerate drops again!
Compelling demonstration of parser combinators for date parsing: https://medium.com/mercury-bank/a-magic-date-input-using-parser-combinators-in-typescript-3c779741bf4c
You build up a series of functions for parsing elements, which composes much more nicely than regexps.