I have a 30k+ character regex that I used to parse US street addresses from a single line of input into component pieces. That’s the largest ( and yes it was crazy... but it just grew and grew...and made sense to me as I decomposed the problem ). That’s the largest one I ever saw.... and it was the fastest way I could solve the problem.. both mentally and processing wise. ( there was some post processing to deal with soundex matching and data base matching of the pieces to confirm the right answer. But it solved the problem with 95%+ accuracy... that I got into the high 99%’s with the post processing. ) So that’s at least an example of an arbitrary problem that was solved with regex. Earl Sent from my iPhone > On Jul 31, 2020, at 11:08 PM, Will Senn wrote: > >  > Oh, and one good google over another, I also found this: > > https://www.drdobbs.com/architecture-and-design/regular-expressions/184410904 > > Which is also great and simple to follow :). > > Will > >> On 7/31/20 7:36 PM, Rob Pike wrote: >> I think this link - https://swtch.com/~rsc/regexp/regexp1.html i- s the best place to start. Superb exposition on the background, theory, and implementation as well as a bit of history of how the industry lost its way with regular expressions. >> >> Regular expressions are beautiful, simple, and widely misunderstood. >> >> -rob >> >> >> On Sat, Aug 1, 2020 at 10:03 AM Bakul Shah wrote: >>> On Jul 31, 2020, at 3:57 PM, Will Senn wrote: >>> > >>> > I've always been intrigued with regexes. When I was first exposed to them, I was mystified and lost in the greediness of matches. Now, I use them regularly, but still have trouble using them. I think it is because I don't really understand how they work. >>> > ... >>> > 1. What's the provenance of regex in unix (when did it appear, in what form, etc)? >>> > 2. What are the 'best' implementations throughout unix (keep it pre 1980s)? >>> > 3. What are some of the milestones along the way (major changes, forks, disagreements)? >>> > 4. Where, in the source, or in a paper, would you point someone to wanting to better understand the mechanics of regex? >>> >>> Start here: https://en.wikipedia.org/wiki/Thompson%27s_construction >>> >>> [I learned about regular expressions in an automata theory class, >>> before I knew anything about Unix. What helped me was learning >>> about finite state machines. You won't need more than paper and >>> pencil to construct one. Reading source code would make more >>> sense once you grasp how to construct a FSM corresponding to a RE.] > > > -- > GPG Fingerprint: 68F4 B3BD 1730 555A 4462 7D45 3EAA 5B6D A982 BAAF