Pretty nice minimal grammar notation. Simple, but not very compact.

ABNF[0] is less beautiful but a bit faster to read and write, mostly thanks to having syntax for explicit repetition.

[0] https://en.wikipedia.org/wiki/Augmented_Backus%E2%80%93Naur_...

I first noticed this grammar format on the "json grammar on the back of a business card". It always impressed me. I appreciate its minimalism.