what: the most complicated thing (that I've actually used in other projects) was a data-driven precedence-climbing parser (which, since sed is usually lousy at dealing with place-value numerics, required sussing out sed-friendly representations of the core data types).

why: because given a "weird machine" that is nearly omnipresent (in the back end, anyway) and is also a Perlis language, who among us would not wish to program it? (and who among us would willingly admit that 1959's https://en.wikipedia.org/wiki/IBM_1620#Model_I programmers had had better code-fu?)

    I must go down to the sed(1) again, to the pattern space and the hold,
    And all I ask is a t branch and a s/ub/st/ to map and fold;
    And the Jolt’s kick and the keys' clack and the write lines breaking,
    And a green glint in mirror shades, and a grey* dawn waking.

* "the color of television, tuned to a dead channel"; no Ἠὼς Ῥοδοδάκτυλος here!