The Nineteenth Byte

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Neil

10:17 PM

The challenge that caused me to find this site was a popcon

DLosc

@Neil Which one?

emanresu A

Challenge idea: Create a regex where it's not known whether a match exists

hyper-neutrino

could be a CnR maybe? challenge is to create an unknown regex, and opposite is to find a match / prove no match can exist

wait can you give an example of what a regex with no match would even be

DLosc

(?!a)a, trivially

emanresu A

Q: Shortest unmatchable regular expression

Your mission is to write the shortest valid regular expression that no string can match, empty string included. Submissions must have this form ("literal notation"): /pattern/optional-flags Shortest regexp wins. The regexp size is counted in characters. (including slashes and flags) Please e...

hyper-neutrino

10:23 PM

ah I see

yeah I am not familiar enough with whatever those ((?!a)) are called

DLosc

Negative lookaheads

But yeah, there's quite a variety of approaches on the challenge emanresu A posted

emanresu A

"make sure that after/before this, there is anything but a"

hyper-neutrino

reminds me I've been meaning to actually get good at regex at some point in time

emanresu A

I forget whcih

DLosc

(?!) looks to the right of the current position (lookahead); (?<!) looks to the left (lookbehind)

hyper-neutrino

10:26 PM

makes sense

DLosc

Similarly (?=) and (?<) for positive lookahead/behind

The asymmetry of the notation bugs me, tho

hyper-neutrino

yeah that is a bit odd

emanresu A

Isn't there also (?<=)?

DLosc

Whoops, yeah, that's what it is

Not (?<)

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