[ruby-core:99716] [Ruby master Bug#17030] Enumerable#grep{_v} should be optimized for Regexp

Issue #17030 has been updated by Dan0042 (Daniel DeLorme).


What about this?

```ruby
2.times do
  p $~  # depends on match *below*
  rx =~ str
end
```

Now imagine if `2.times` is replaced by `foo`; a priori we can't know if or how many times the block will be executed. So what I was trying to say is that flow control can lead to all kinds of code paths where it's extremely difficult to know which matching operations a pseudo-global may depend on. Maybe not impossible, but personally I wouldn't want to code that kind of analysis when a simple approach is enough for >90% of cases, and guaranteed to be bug-free.


> There will be other false positives: `str.gsub(regexp, &block)`. That's not a real issue, simply assume that `block` will want access to `Regexp.last_match`.

Actually... `block` does not have access to `Regexp.last_match` (unless you created the block in the same scope as the gsub operation, but that would be unusual)

----------------------------------------
Bug #17030: Enumerable#grep{_v} should be optimized for Regexp
https://bugs.ruby-lang.org/issues/17030#change-87206

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Currently:

```ruby
array.select { |e| e.match?(REGEXP) }

# about 3x faster and 6x more memory efficient than
array.grep(REGEXP)
```

This is because `grep` calls `Regexp#===` which creates useless `MatchData`



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