[TUHS] Nice video with Brian Kernighan

Mohamed Akram mohd.akram at outlook.com
Sun Aug 21 19:36:03 AEST 2022

Hi folks,

This is my first time posting on this list, it’s been such a joy to read about so many little-known yet enduring and consequential aspects of UNIX. I had written a short post [1] about the calendar utility some time ago, with a brief glimpse at its history. Seeing its implementation, it certainly made me scratch my head a bit - who would think to create a program whose sole purpose was to dynamically generate a regular expression that would then be fed into another program (it doesn’t stop there either, as my post goes into). I found it to be perhaps the most illustrative and comprehensive example of UNIX composition that I had come across. Unbeknownst to me that it was Douglas McIlroy who had written this program, which in hindsight should not come as a surprise at all, him being the exemplar of composing simple, orthogonal, yet robust tools to get a job done quickly and efficiently.

Thank you Doug for your reply, I thoroughly enjoyed learning more about the origins and history of the calendar program. That it was the impetus to turn egrep into the performant and viable tool that we know today further colors the picture of this unassuming utility.

[1] https://akr.am/blog/posts/today-in-history-brought-to-you-by-unix


On Aug 20, 2022, at 7:48 PM, Douglas McIlroy <douglas.mcilroy at dartmouth.edu<mailto:douglas.mcilroy at dartmouth.edu>> wrote:

Brian's tribute to the brilliant regex mechanism that awk borrowed
from egrep  spurred memories.

For more than forty years I claimed credit for stimulating Ken to
liberate grep from ed. Then, thanks to TUHS, I learned that I had
merely caused Ken to spring from the closet a program he had already
made for his own use.

There's a related story for egrep. Al Aho made a deterministic
regular-expression recognizer as a faster replacement for the
non-deterministic recognizer in grep. He also extended the domain of
patterns to full regular expressions, including alternation; thus the
"e" in egrep.

About the same time, I built on Norm Shryer's personal calendar
utility. I wanted to generalize Norm's strict syntax for dates to
cover most any (American) representation of dates, and to warn about
tomorrow's calendar as well as today's--where "tomorrow" could extend
across a weekend or holiday.

Egrep was just the tool I needed for picking the dates out of a
free-form calendar file. I wrote a little program that built an egrep
pattern based on today's date. The following mouthful for Saturday,
August 20 covers Sunday and Monday, too. (Note that, in egrep, newline
is a synonym for |, the alternation operator.)

       (^|[ (,;])(([Aa]ug[^ ]* *|(08|8)/)0*20)([^0123456789]|$)
       (^|[ (,;])(([Aa]ug[^ ]* *|(08|8)/)0*21)([^0123456789]|$)
       (^|[ (,;])(([Aa]ug[^ ]* *|(08|8)/)0*22)([^0123456789]|$)

It worked like a charm, except that it took a good part of a minute to
handle even a tiny calendar file. The reason: the state count of the
deterministic automaton was exponentially larger than the regular
regular expression; and egrep had to build the automaton before it
could run it. Al was mortified that an early serious use of egrep
should be such a turkey.

But Al was undaunted. He replaced the automaton construction with an
equivalent lazy algorithm that constructed a state only when the
recognizer was about to visit it. This made egrep into the brilliant
tool that Brian praised.

What I don't know is whether the calendar program stimulated the idea
of lazy implementation, or whether Al, like Ken before him with grep,
already had the idea up his sleeve.


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