Issue #21859 has been updated by tompng (tomoya ishida). First issue
This should be false in both cases.
I think `Regexp.linear_time?(/(?<=(a))/)` matches in linear time. If the issue is just for inconsistency between lookahead and lookbehind, it's not a bug. Here's an example: ~~~ruby Regexp.linear_time?(/x+(?=(a))/) #=> false Regexp.linear_time?(/x+(?<=(a))/) #=> true /x+(?=(a))/.match?('x' * 100000) #=> processing time: 28.599804s not linear_time /x+(?<=(a))/.match?('x' * 100000) #=> processing time: 0.016630s linear_time ~~~ Second issue: `/(?!(a))b/` `/(?<!(a))b/`
I believe such a capture group can never capture anything
Capture group in negative lookahead can capture and can be used inside negative lookahead. For negative lookbehind, I think it's just a restriction of onigmo. ~~~ruby regexp = /(?!([a-z])\1)[a-z]{2}/ regexp.match?('ab') #=> true regexp.match?('aa') #=> false ~~~ ---------------------------------------- Bug #21859: Inconsistent behaviors in Regexp lookbehind/lookahead with capture groups https://bugs.ruby-lang.org/issues/21859#change-116259 * Author: trinistr (Alexander Bulancov) * Status: Open * ruby -v: ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x86_64-linux] * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN ---------------------------------------- First issue: `Regexp.linear_time?` is `false` for a positive lookahead with a capture, but `true` for a positive lookbehind: ```ruby irb(main):002> Regexp.linear_time?(/(?=(a))/) => false irb(main):003> Regexp.linear_time?(/(?<=(a))/) => true ``` This should be `false` in both cases. Second issue: Capture group is allowed in a negative lookahead, but causes a `SyntaxError` in a negative lookbehind: ```ruby irb(main):001> /(?!(a))b/ => /(?!(a))b/ irb(main):002> /(?<!(a))b/ /home/alex/.local/share/mise/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/irb-1.16.0/exe/irb:9:in '<top (required)>': (irb):2: invalid pattern in look-behind: /(?<!(a))b/ (SyntaxError) ``` I believe such a capture group can never capture anything, so it probably should be an error in both cases. -- https://bugs.ruby-lang.org/