[JonChesterfield]

Appreciated, reading through it. I suspect the majority of the sed experience can be attributed to it using "posix regular expressions" by default. It was about a decade after first discovering sed that I realised passing -E was really important.

It is difficult for newcomers to guess that "extended regular expressions" refers to the barely-usable subset of "regular expressions" and "posix regular expressions" are terrible in comparison to either.

edit: alright, yes, one can program in that. Sed can recurse.

  .PHONY: help3
  help3:
   sed -nE 's/^([a-zA-Z_*.-]+):.*## (.*)$$/\1 :\2/ p' \
   $(MAKEFILE_LIST) | \
   sed -E -e ':again s/^([^:]{1,16})[:]([^:]+)$$/\1 :\2/ ' -e 't again '  |\
   sed -E 's/^([^ ]*)([ ]*):(.*)$$/\1:\2\3/' |\
   sort

The first invocation filters out the lines of interest, second one space pads to 16. That works by putting the colon before the help text and repeatedly inserting a space before the colon until there are at least sixteen non-colon characters in the first group.

Composing the -n/p combination with act on everything is a stumbling block for merging the multiple invocations together but I expect it to be solvable.