this post was submitted on 31 Oct 2024
330 points (98.0% liked)

Programmer Humor

32473 readers
420 users here now

Post funny things about programming here! (Or just rant about your favourite programming language.)

Rules:

founded 5 years ago
MODERATORS
 

^.?$|^(..+?)\1+$

Matches strings of any character repeated a non-prime number of times

https://www.youtube.com/watch?v=5vbk0TwkokM

you are viewing a single comment's thread
view the rest of the comments
[–] fubo 135 points 2 weeks ago* (last edited 2 weeks ago) (17 children)

The answer given in the spoiler tag is not quite correct!

Test caseAccording to the spoiler, this shouldn't match "abab", but it does.

Corrected regexThis will match what the spoiler says: ^.?$|^((.)\2+?)\1+$

Full workupAny Perl-compatible regex can be parsed into a syntax tree using the Common Lisp package CL-PPCRE. So if you already know Common Lisp, you don't need to learn regex syntax too!

So let's put the original regex into CL-PPCRE's parser. (Note, we have to add a backslash to escape the backslash in the string.) The parser will turn the regex notation into a nice pretty S-expression.

> (cl-ppcre:parse-string "^.?$|^(..+?)\\1+$")
(:ALTERNATION
 (:SEQUENCE :START-ANCHOR (:GREEDY-REPETITION 0 1 :EVERYTHING) :END-ANCHOR)
 (:SEQUENCE :START-ANCHOR
  (:REGISTER
   (:SEQUENCE :EVERYTHING (:NON-GREEDY-REPETITION 1 NIL :EVERYTHING)))
  (:GREEDY-REPETITION 1 NIL (:BACK-REFERENCE 1)) :END-ANCHOR))

At which point we can tell it's tricky because there's a capturing register using a non-greedy repetition. (That's the \1 and the +? in the original.)

The top level is an alternation (the | in the original) and the first branch is pretty simple: it's just zero or one of any character.

The second branch is the fun one. It's looking for two or more repetitions of the captured group, which is itself two or more characters. So, for instance, "aaaa", or "abcabc", or "abbaabba", but not "aaaaa" or "abba".

So strings that this matches will be of non-prime length: zero, one, or a multiple of two numbers 2 or greater.

But it is not true that it matches only "any character repeated a non-prime number of times" because it also matches composite-length sequences formed by repeating a string of different characters, like "abcabc".

If we actually want what the spoiler says — only non-prime repetitions of a single character — then we need to use a second capturing register inside the first. This gives us:

^.?$|^((.)\2+?)\1+$.

Specifically, this replaces (..+?) with ((.)\2+?). The \2 matches the character captured by (.), so the whole regex now needs to see the same character throughout.

[–] taiyang 10 points 2 weeks ago (2 children)

Thanks, I now have insight into my own personal hell for when I die.

[–] fubo 11 points 2 weeks ago (1 children)

Regex is good for a few very specific things, and sysadmins used to use it for goddamn everything. If all your server logs are in lightly-structured text files on a small number of servers, being able to improvise regex is damn useful for tracking down server problems. Just write a shell loop that spawns an ssh logging into each server and running grep over the log files, to look for that weird error.

These days, if you need to crunch production server logs you probably need to improvise in SQL and jq and protobufs or systemd assmonkery or something.

But if you actually need a parser, for goodness sake use a parser combinator toolkit, don't roll your own, especially not with regex. Describing your input language in plain Haskell is much nicer than kludging it.

(This is the "totally serious software engineering advice" forum, right?)

[–] taiyang 2 points 2 weeks ago

I've worked mostly as a data scientist / analyst but regex was being user to identify various things in the SQL database (which was viewed locally via R table). I forget the exact is cases, mostly remembering how complex some of it got... Especially after certain people were using GPT to build them.

And GPT like to make up extra bits not necessary, but my coworkers didn't exactly have the knowledge to read regex, which lead to nobody really checking it. Now it just gives me anxiety, haha.

load more comments (14 replies)