From: "jeremyevans0 (Jeremy Evans)" Date: 2021-10-13T16:43:28+00:00 Subject: [ruby-core:105633] [Ruby master Bug#17774] Quantified empty group causes regex to fail Issue #17774 has been updated by jeremyevans0 (Jeremy Evans). I looked into fixing this by removing the define of `USE_MONOMANIAC_CHECK_CAPTURES_IN_ENDLESS_REPEAT`, as @mame indicated: https://github.com/ruby/ruby/commit/018922ba15eb7aea86957789d7defae9ffc43688 It ends up breaking a few specs. For example, it changes the behavior of: ```ruby /(a|\2b|())*/.match("aaabbb").to_a # Before: # => ["aaabbb", "", ""] # After: # => ["aaa", "", ""] ``` For this example, Ruby 1.8 returns `["aaa", "a", nil]`. The equivalent in Perl returns `["aaa", "", ""]`. The equivalent in Python 2 and 3 returns `["aaabbb", "", ""]`. I think the `["aaabbb", "", ""]` result seems best for a greedy match since it matches the most characters. However, I can also see where an implementation would return one of the other results if a scan terminates when no forward progress is made during an iteration. Anyway, if we are OK with this behavior change for empty capture groups, I can submit the commit as a pull request. However, I think it would be better to wait for a fix in Onigmo. ---------------------------------------- Bug #17774: Quantified empty group causes regex to fail https://bugs.ruby-lang.org/issues/17774#change-94123 * 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: