Doug, No matter. I have often thought about what an amazing muse you were to so many people regarding so many different ideas that have panned out. It seems to me that you were always there at the right time. Your powers to get the best from everyone around you are unlike anyone else I have ever been lucky enough to have met. Although I have also often said to students the one really hard thing to directly teach, that you can learn by looking at people that came before you is 'good taste.' We all owe you as much thanks for being there and inspiring your peers, as for their brilliance in implementing the concepts with style and taste. Clem ᐧ On Sat, Aug 20, 2022 at 11:49 AM Douglas McIlroy < douglas.mcilroy(a)dartmouth.edu&gt; wrote:

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 Regards, Mohamed On Aug 20, 2022, at 7:48 PM, Douglas McIlroy <douglas.mcilroy@dartmouth.edu<mailto:douglas.mcilroy@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. Doug