^.?$|^(..+?)\1+$
<answer>
Matches strings of any character repeated a non-prime number of times
I’m going to assume the answer is a magic square attempt that just isn’t very good
No cookie for me I just tried it in Notepad++ and VS code and it matches lines of one characer (first group I think) or the starting of a line that is an at least 2 characters string repeated twice (second group it seems)
so the second group matches abab
abcabc abcdeabce abcdefabcdefNothing about prime numbers really only first repetition gets a match. Very interesting Honestly I used regex from years and never had to retort to something like this ever. I can only imagine it useful to check for a password complexity to not be repeated strings like I do for sites that I just want in and use a yopmail.com mail to register a fake user.
All my homies hate regexs. That’s actually the best use case I found for LLMs so far : I just tell it what I want it to match or not match, and it usually spits out a decent one
Relevant xkcd:
Empty input Or input of exactly 1 character Or input of at least 2 characters, followed by at least 1 something (idk what \1 matches)
Did I get it (almost)?
The answer given in the spoiler tag is not quite correct!
Test case
According to the spoiler, this shouldn’t match “abab”, but it does.
Corrected regex
This will match what the spoiler says:
^.?$|^((.)\2+?)\1+$Full workup
Any 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
\1and 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 “ababab”, or “abbabba”, 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\2matches the character captured by(.), so the whole regex now needs to see the same character throughout.“abbabba”
“abbabba” doesn’t match the original regex but “abbaabba” does
I upvoted this because I hate it.
Whatever you do, don’t get in a time machine back to 1998 and become a Unix sysadmin.
(Though we didn’t have CL-PPCRE then. It’s really the best thing that ever happened to regex.)
I have to admit that using CL-PPCRE does not really help me understanding the regexp any better. But this may be because I deal with complex regexps for decades now, and I just read them.
I upvoted you because I consider Perl write only (used to know it, now it inspires readable code as a high priority)
Let’s put it this way: You can produce unreadable code in basically any language. With Perl, it is just a bit easier.
And of course if you have the discipline of a good programmer, even your casual Perl programs should be readable. That’s what differenciates a good programmer from a hacker.
Yeah, I was younger then, perhaps less disciplined, and as always, given enough work you can decompile or regenerate anything. Still, I contend, the nature of Perl, powerful as it was, lent itself to unmaintainable messes, and I’m not talking regex’s, which is why it has faded, no amount of get gud withstanding.
Thanks, I now have insight into my own personal hell for when I die.
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
sshlogging into each server and runninggrepover 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
jqand 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?)
no
that is correct!
Syntactically valid Perl
Something like
!“A line with exactly 0 or 1 characters, or a line with a sequence of 1 or 3 or more characters, repeated at least twice”!<
It’s a line with a sequence of two or more characters repeated at least twice.
Only the part after the pipe character. The pipe character works as an “or” operator. RegalPotoo is right.
They said—
A line with exactly 0 or 1 characters, or a line with a sequence of 1 or 3 or more characters, repeated at least twice
Note—
…or a line with a sequence of 1 or 3 or more characters, repeated at least twice
It should be—
…or a line with a sequence of 2 or more characters, repeated at least twice
The regex in the post will match “abab”. Their original description (line 2 of this comment) will not match “abab”.
I agree, you’re right about the part after the pipe and RegalPotoo’s explanation was not entirely correct.
Just waiting for the oppertunity to hide this in prod.
It matches “yo momma”.
Looks like APL to me.
Hot take: You’re shit at coding if you can’t do regex.
Regular expressions in general, and automata theory, sure you should know about that. But a specific extended regex language like here? That’s like saying you’re shit at coding if you can’t do <insert arbitrary programming language here>.
Ah yes, spoken like a true elitist asshole
It takes a couple of hours to learn the basics.
And then a few more any time you actually want to use it.
And then double it each time you have to decipher the existing one
Just don’t use regex unless there is really no other way, and when you absolutely have to - frankly, that’s one of the ultra rare occasions I recommend using the AI.
You recommend using AI to produce code you don’t understand?
That I do, yes, because that’s a small chunk of code that - when necessary - would have to be completely remade anyways, not just modified.
That wouldn’t fly during a code review.
That’s your opinion my man
I’m not gonna continue using arguments if all you can respond with is cynicism, apparently I wasn’t wrong about the elitism part
Regex should generally be avoided wherever possible.
Yeah but it’s just so tempting… It validates so many inputs so easily…
And misses others you didn’t think about.
Yeah, I’ve found myself wasting quite a lot of time thinking of the ‘perfect regex’ for task X only to realise that I could have avoided doing so by simply taking a different approach.
