[ruby-dev:52218] [Ruby Bug#21933] Ruby::Box: named capture local variable can become nil after non-matching lines

11 May 2026

      Issue #21933 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED, 4.0: REQUIRED to 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED, 4.0: DONE

ruby_4_0 commit:dd78605b2d06600750c331f307083d60df702814 merged revision(s) commit:526344b56ea968d5704bdefe6e10bb3cf7f4f569, commit:8ad6baa01746e8de0460f0ccdaee69953a70af17.

----------------------------------------
Bug #21933:  Ruby::Box: named capture local variable can become nil after non-matching lines
https://bugs.ruby-lang.org/issues/21933#change-117268

* Author: katsyoshi (Katsuyoshi MATSUMOTO)
* Status: Closed
* ruby -v: 4.0.1
* Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED, 4.0: DONE
----------------------------------------
Environment:
- Ruby 4.0.1
- RUBY_BOX=1
- OS: Linux x86_64

Summary:
With Ruby::Box enabled, a named capture local variable from `=~` may become nil,
even when the regexp matches. This happens after iterating over non-matching lines first.

Reproducer:

sample.rb
``` ruby

text = <<~ASM
  .intel_syntax noprefix
  .globl main
main:
  nop
ASM

cur=nil
text.each_line do |line|
  if /^(?<label>[_A-Za-z.]\w*):/ =~ line
    p [:matched, line.inspect, label.inspect]
    cur = label
    break
  end
end
p [:cur, cur.inspect]
```

Expected:
``` shell
[:matched, "\"main:\\n\"", "\"main\""]
[:cur, "\"main\""]
```

Actual (with RUBY_BOX=1):
``` shell
[:matched, "\"main:\\n\"", "nil"]
[:cur, "nil"]

```

Control:
Without RUBY_BOX=1, the same script returns "main" as expected.

-- 
https://bugs.ruby-lang.org/