From: "alanwu (Alan Wu) via ruby-core" <ruby-core@...>
Date: 2025-11-07T22:43:44+00:00
Subject: [ruby-core:123725] [Ruby Bug#13671] Regexp with lookbehind and case-insensitivity raises RegexpError only on strings with certain characters

Issue #13671 has been updated by alanwu (Alan Wu).

Backport changed from 3.2: REQUIRED, 3.3: DONE, 3.4: REQUIRED to 3.2: REQUIRED, 3.3: DONE, 3.4: DONE

ruby_3_4 commit:3150a1d989d81089c7da7d0491321e370e71f482

----------------------------------------
Bug #13671: Regexp with lookbehind and case-insensitivity raises RegexpError only on strings with certain characters
https://bugs.ruby-lang.org/issues/13671#change-115111

* Author: dschweisguth (Dave Schweisguth)
* Status: Closed
* Assignee: duerst (Martin D��rst)
* ruby -v: 2.4.1
* Backport: 3.2: REQUIRED, 3.3: DONE, 3.4: DONE
----------------------------------------
Here is a test program:

~~~ ruby
def test(description)
  begin
    yield
    puts "#{description} is OK"
  rescue RegexpError
    puts "#{description} raises RegexpError"
  end
end

test("ass, case-insensitive, special") { /(?<!ass)/i =~ '���' }
test("bss, case-insensitive, special") { /(?<!bss)/i =~ '���' }
test("as,  case-insensitive, special") { /(?<!as)/i  =~ '���' }
test("ss,  case-insensitive, special") { /(?<!ss)/i  =~ '���' }
test("ass, case-sensitive,   special") { /(?<!ass)/  =~ '���' }
test("ass, case-insensitive, regular") { /(?<!ass)/i =~ 'x' }

~~~

Running the test program with Ruby 2.4.1 (macOS) gives

~~~
ass, case-insensitive, special raises RegexpError
bss, case-insensitive, special raises RegexpError
as,  case-insensitive, special is OK
ss,  case-insensitive, special is OK
ass, case-sensitive,   special is OK
ass, case-insensitive, regular is OK

~~~

The RegexpError is "invalid pattern in look-behind: /(?<!ass)/i (RegexpError)"

Side note: in the real code in which I found this error I was able to work around the error by using (?i) after the lookbehind instead of //i.

Running the test program with Ruby 2.3.4 does not report any RegexpErrors.

I think this is a regression, although I might be wrong and it might be saving me from an incorrect result with certain strings.

---Files--------------------------------
test.rb (531 Bytes)


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