From: "trinistr (Alexander Bulancov) via ruby-core" <ruby-core@...>
Date: 2026-02-03T16:11:44+00:00
Subject: [ruby-core:124666] [Ruby Bug#21859] Inconsistent behaviors in Regexp lookbehind/lookahead with capture groups

Issue #21859 has been updated by trinistr (Alexander Bulancov).


> I think `Regexp.linear_time?(/(?<=(a))/)` matches in linear time.

I apologize, it seems I got distracted and forgot to actually check the execution times.
But this is interesting behavior. Maybe constant-size lookahead can be optimized to also be linear? It seems strange to me that these cases are so similar but behave very differently.

> Capture group in negative lookahead can capture and can be used inside negative lookahead.

I've not been able to find a case where the capture group actually captures, not just overall regexp matches. Isn't it impossible? To match, regexp needs to satisfy *negative* lookahead, so there should *not be* anything to capture.

```ruby
regexp = /(?!([a-z])\1)[a-z]{2}/
regexp.match('ab') # => #<MatchData "ab" 1:nil>
regexp.match('aabaa') # => #<MatchData "ab" 1:nil>

regexp = /[a-z]{2}(?!([a-z])\1)/
regexp.match('aabaa') # => #<MatchData "aa" 1:nil>
```

----------------------------------------
Bug #21859: Inconsistent behaviors in Regexp lookbehind/lookahead with capture groups
https://bugs.ruby-lang.org/issues/21859#change-116261

* 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.



-- 
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