[ruby-core:107892] [Ruby master Bug#18631] Range check breaks multiplex backreferences in regular expressions

Issue #18631 has been reported by jirkamarsik (Jirka Marsik).

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Bug #18631: Range check breaks multiplex backreferences in regular expressions
https://bugs.ruby-lang.org/issues/18631

* Author: jirkamarsik (Jirka Marsik)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.5p203 (2021-11-24) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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The regular expression engine can sometimes produce wrong results when using multiplex backreferences near the end of the input string.

``` ruby
irb(main):001:0> /(?<x>a)(?<x>aa)\k<x>/.match("aaaaa")
=> #<MatchData "aaaaa" x:"a" x:"aa">
irb(main):002:0> /(?<x>a)(?<x>aa)\k<x>/.match("aaaa")
=> nil
irb(main):003:0> /(?<x>a)(?<x>aa)\k<x>/.match("aaaab")
=> #<MatchData "aaaa" x:"a" x:"aa">
```

The second and third calls to `match` should produce the same result.

The cause is the `DATA_ENSURE(n)` macro in the `OP_BACKREF_MULTI` case in `regexec.c` (https://github.com/ruby/ruby/blob/master/regexec.c#L2646). Instead of using `continue` to try to match the other referents for the backref (as all the other branches do), the `DATA_ENSURE` macro uses `goto fail` and so skips the other referents of the multiplex backref. This means that after failing the range check, no other referent can match. By extending the input string in the third example above, we have avoided the bug and got the correct result.



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