[TUHS] Regular Expressions

ron minnich rminnich at gmail.com
Sat Aug 1 14:53:00 AEST 2020


kernighan did this one at a talk at udel in 1976: find all the words
in the dictionary that you can see on a calculator looking at it
upside down.

On Fri, Jul 31, 2020 at 9:32 PM Earl Baugh <earl.baugh at gmail.com> wrote:
>
> 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 <will.senn at gmail.com> 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 <bakul at iitbombay.org> wrote:
>>
>> On Jul 31, 2020, at 3:57 PM, Will Senn <will.senn at gmail.com> 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


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