From: mame@... Date: 2021-04-05T06:16:15+00:00 Subject: [ruby-core:103236] [Ruby master Bug#17774] Quantified empty group causes regex to fail Issue #17774 has been updated by mame (Yusuke Endoh). Thank you, I can reproduce the issue. The issue is in the code from [onigmo](https://github.com/k-takata/Onigmo), so it would be helpful if you could report this issue to the upstream. By a quick investigation, an optimization expands `(){4}`, and does not expand `(){5}`, which makes the difference of the behavior. Enabling debug output suggests that the bug is caused by `USE_MONOMANIAC_CHECK_CAPTURES_IN_ENDLESS_REPEAT` option. The `(){5}` case works great by the following change that disables the option, but I'm unsure the performance impact. ``` diff --git a/regint.h b/regint.h index 0740429688..968ea6cde8 100644 --- a/regint.h +++ b/regint.h @@ -71,7 +71,6 @@ #define USE_PERL_SUBEXP_CALL #define USE_CAPITAL_P_NAMED_GROUP #define USE_BACKREF_WITH_LEVEL /* \k, \k */ -#define USE_MONOMANIAC_CHECK_CAPTURES_IN_ENDLESS_REPEAT /* /(?:()|())*\2/ */ #define USE_NEWLINE_AT_END_OF_STRING_HAS_EMPTY_LINE /* /\n$/ =~ "\n" */ #define USE_WARNING_REDUNDANT_NESTED_REPEAT_OPERATOR /* !!! moved to regenc.h. */ /* #define USE_CRNL_AS_LINE_TERMINATOR */ ``` ---------------------------------------- Bug #17774: Quantified empty group causes regex to fail https://bugs.ruby-lang.org/issues/17774#change-91317 * Author: Davidebyzero (David Ellsworth) * Status: Open * Priority: Normal * ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-msys] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN ---------------------------------------- The regex `^((x*)(?=\2$))*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: