From: s.wanabe@... Date: 2021-05-01T20:06:55+00:00 Subject: [ruby-core:103689] [Ruby master Bug#17774] Quantified empty group causes regex to fail Issue #17774 has been updated by wanabe (_ wanabe). sawa (Tsuyoshi Sawada) wrote in #note-3: > wanabe (_ wanabe) wrote in #note-2: > > ... Oniguruma, a derivative of Onigmo > > I believe it is the other way around. Oh I'm very sorry, I wrote it wrong. I was aware of it, but I simply used the wrong word. ---------------------------------------- Bug #17774: Quantified empty group causes regex to fail https://bugs.ruby-lang.org/issues/17774#change-91782 * Author: Davidebyzero (David Ellsworth) * Status: Open * Priority: Normal * ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-msys] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN ---------------------------------------- The regex `^((x*)(?=\2$))*x$` matches powers of 2 in unary, expressed as strings of `x` characters whose length is the number. Adding an empty group `()` in the middle of it should have no effect on its operation, and indeed it does not. `^((x*)()(?=\2$))*x$` still matches powers of 2 just fine. Quantifying that empty group, `(){4}`, should still have no effect. And indeed, `^((x*)(){4}(?=\2$))*x$` still matches powers of 2. But quantify that to `(){5}`, and suddenly it fails. The following command line should print `1`, but instead prints nothing: ``` ruby -e 'print 1 if "x"*32 =~ /^((x*)(){5}(?=\2$))*x$/' ``` However this one does print `1`: ``` ruby -e 'print 1 if "x"*32 =~ /^((x*)(){4}(?=\2$))*x$/' ``` Bug found to occur on [Try It Online](https://tio.run/): `ruby 2.5.5p157 (2019-03-15 revision 67260) [x86_64-linux]` Bug confirmed to happen on my own machine: `ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-msys]` Solving the challenge [Is that number a Two Bit Number������?](https://codegolf.stackexchange.com/questions/211840/is-that-number-a-two-bit-number%ef%b8%8f/222792#222792) on Code Golf Stack Exchange is what led me to discover this bug. -- https://bugs.ruby-lang.org/ Unsubscribe: