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12:00 AM
@Grimy Awesome! The thing that impresses me the most is that the inequality is precisely equivalent; it's not even off by one, even though there is some wiggle room in which it still makes an overall robust regex. Were you aware that this "power of 2 minus 1" trick was previously used in Is it a Mersenne Prime??
 
 
6 hours later…
5:39 AM
Fixed the anchor bug in RME.
 
 
1 hour later…
7:08 AM
I hadn’t seen Mersenne Prime, but I’ve used power of 2 minus 1 (and power of 2 minus 2, which is the same length) in my log2 regexes
Good work squashing that bug (=
 
7:28 AM
Oh also the variant you used in Mersenne Prime is suboptimal
 
It's not "wrong", it just doesn't match 0
And hi :)
 
It should be (x(x+)(?=\3$))+ instead of It (x(x+)(?=\3$))*x
For -1 byte
Hi (=
 
Yes, I realized that just before you said it
It actually still needs to be 31 bytes, but this fixes the incorrect match of 1 it had.
 
7:52 AM
Alternative 31: ^(?!(xx+)\1+$|((xx)+)(\2x)*$)xx
Or without the final xx, it’s a 29 that incorrectly matches 0 and 1
 
@Grimy Cool, I was thinking along the same lines, but didn't go as far as coming up with a negative that matches powers of 2 minus 1. Very cool that it came out to be 31.
 
8:12 AM
Here’s a slow 28 that incorrectly matches 0, 1, 2, and 4 ^(?!(x?)(xx\1*(xx)*)(\2x)+$)
Oh actually it has infinitely many false positives (all powers of 2 that are 1 less than a prime)
Not very good
 
@Grimy If that's true, why doesn't it match 256? That's one less than a prime.
 
Oh well, I just don’t understand how it works then
 
And 16
What false positives did you find?
 
8:28 AM
0, 1, 2, 4 I found by testing
"all powers of 2 1 less than a prime" I found by (apparently bad) reasoning
Also this 26 is equivalent ^(?!(x?)((xx)+\1*)(\2x)+$), so if 0, 1, 2, 4 are the only false positives it’s maybe fixable in ≤ 31
Ooh and \1? instead of \1* is still equivalent and considerably faster
Working 34: ^(?!(x?)((xx)+\1?)(?=\1)(\2x)*$)xx
 
 
4 hours later…
12:48 PM
@Grimy all sets in order of regex shortness.html - another project in the vein of "busy beavers", but probably more interesting, and using standard ECMA.
 
1:42 PM
@Grimy 26: ^(?!(xx+|(x(x))+)(\1\3)+$)xx (Took me longer than I care to admit to arrive at that)
cc @Deadcode
 
@H.PWiz Very nice try, but it has an incorrect match of 2
^(?!(xx+|(x(x))+)(\1\3)+$)xxx is still 27 bytes...
 
Alright, xxx at the end
I miscounted, ^(?!(xx+|(x(x))+)(\1\3)+$)xxx is 29
 
That's still 2 bytes better...
 
yes
 
@H.PWiz The proof of this is not as obvious as that of Grimy's 26/34... have you proven it?
It's effectively an AND between ^(?!(xx+)\1+$)xxx and ^(?!((xx)+)(x\1)+$), but ^(?!((xx)+)(x\1)*$) is what matches powers of 2 minus 1
 
1:54 PM
So, (?!((xx)+)(x\1)+$) works the same as ^(?!((xx)+)(x\1)*$) except that it doesn't reject numbers of the form (xx)+, even numbers. Of which 2 is the only prime
 
Oooh, awesome. Very nicely done! Are you going to post it as an answer?
 
yeah, soon
 
 
7 hours later…
8:53 PM
Great job on the 29 (=
 

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