[ruby-core:115979] [Ruby master Bug#20104] Regexp#match returns nil but allocates T_MATCH objects

["luke-gru (Luke Gruber) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20104 has been updated by luke-gru (Luke Gruber).


So I looked into this and it's this commit that introduces that behavior: 7193b404a1a56e50f8046d0382914907020c1559

----------------------------------------
Bug #20104: Regexp#match returns nil but allocates T_MATCH objects
https://bugs.ruby-lang.org/issues/20104#change-105942

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.4.0dev (2023-12-30T03:14:38Z master 8e32c01742) [x86_64-openbsd7.4]
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED
----------------------------------------
Between Ruby 3.2 and 3.3, behavior changed so that Regexp#match will allocate a T_MATCH object even when there is no match.  Example code:

```ruby
h = {}
GC.start
GC.disable
ObjectSpace.count_objects(h)
matches = h[:T_MATCH] || 0
md = /\A[A-Z]+\Z/.match('1')
ObjectSpace.count_objects(h)
new_matches = h[:T_MATCH] || 0
puts "/\\A[A-Z]+\\Z/.match('1') => #{md.inspect} generates #{new_matches - matches} T_MATCH objects"
```

Result with Ruby 1.9-3.2:

```
/\A[A-Z]+\Z/.match('1') => nil generates 0 T_MATCH objects
```

Results with Ruby 3.3.0 and current master branch:

```
/\A[A-Z]+\Z/.match('1') => nil generates 1 T_MATCH objects
```

This results in a measurable performance decrease for both Sinatra and Roda web applications, as reported at: https://old.reddit.com/r/ruby/comments/18sxtv9/ruby_330_performance_ups_and_downs/

Thanks to GitHub users kiskoza and tagliala for producing a minimal example showing this issue: https://github.com/caxlsx/caxlsx/issues/336



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