Re:The quadratic problem nobody fixed
by mananaysiempre on Tuesday March 24, 2026 @01:30PM
(#47506256)
>> lazy quantifiers for inclusion into the next release of POSIX
> That is surprising!
I mean, POSIX BREs (only!) also include (single-digit) backreferences. Surprisingly, (with such a restriction) this is actually polynomial[1,2] if impractical (h/t 'burntsushi for the reference[3]). But I still wouldn’t take POSIX as the arbiter of sanity in this case. Thus far I’m not even convinced their text actually amounts to a well-defined ordering on parse trees.
I don’t know if nongreedy quantifiers are all that interesting without match groups, though, so this isn’t a particularly burning question in my view.
[1] https://branchfree.org/2019/04/04/question-is-matching-fixed...
That is surprising!
We've found that in certain simpler scenarios it's possible to use complement to express lazy quantifiers, but in the general case it appears very fragile.
a simple example: a.*?b can be rewritten to something like (a.*b&~(a.*b_+)) in RE# syntax, which effectively means "there is a match, but must not contain a shorter match"
> 1998 TOPLAS
I will read through it properly, but i can already see from page 8 that it requires a table of (DFA size x input length) which makes me very suspicious that it is more of a thought exercise than a real world solution.
> It is true that you can’t stream the haystack in the general case, but to what precise extent you can is an interesting question with a known algorithmic answer[3].
Thank you for this, this is an interesting paper