[defrost]

Whilst acknowledging that "Confused by SED" has overlap with "have used SED for 40 years and have three books purely on SED" I can recommend

https://www.grymoire.com/Unix/Sed.html

as a reference some might occasionally swear by.

    Anyhow, sed is a marvelous utility. Unfortunately, most people never learn its real power. The language is very simple, but the documentation is terrible. The Solaris on-line manual pages for sed are five pages long, and two of those pages describe the 34 different errors you can get. A program that spends as much space documenting the errors as it does documenting the language has a serious learning curve.

[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.

[JonChesterfield]

After a slightly dubious use of time, I can confirm that columns is not necessary. Also noticed that the original version missed double_colon:: style targets. I fear sort is not necessary either but one has to draw the line somewhere.

  HELP_PADDING := 30
  
  .PHONY: awkhelp
  awkhelp: ## Write this help using awk
   @echo "awkhelp:"
   @awk 'BEGIN {FS = ":.*#+"}; /^[a-zA-Z_*.-]+:.*## .*$$/ {printf "  %-'$(HELP_PADDING)'s %s\n", $$1, $$2}' \
   $(MAKEFILE_LIST) | \
   sort
  
  .PHONY: sedhelp
  sedhelp: ## Write this help using sed
   @echo "sedhelp:"
   @sed -E \
   -e '/^([a-zA-Z_*.-]+::?[ ]*)##[ ]*([^#]*)$$/ !d # grep' \
   -e 's/([a-zA-Z_*.-]+:):?(.*)/  \1\2/ # drop :: and prefix pad' \
   -e ':again s/^([^#]{1,'$(HELP_PADDING)'})##[ ]*([^#]*)$$/\1 ##\2/ # insert a space' \
   -e 't again # do it again (termination is via {1, HELP_PADDING})' \
   -e 's/^([^#]*)##([^#]*)$$/\1\2/ # remove the ##' \
   $(MAKEFILE_LIST) | \
   sort