[ruby-core:99708] [Ruby master Bug#17030] Enumerable#grep{_v} should be optimized for Regexp

Issue #17030 has been updated by marcandre (Marc-Andre Lafortune).


fatkodima (Dima Fatko) wrote in #note-13:
> In many cases, probably yes, but again, `case-when`, when arguments/consts/etc instead of local vars are used - it is hard to tell if them are regexes or not.

That's not really what I'm proposing. I'm proposing something like an internal `Regexp.needs_last_match?` that would return `true` or `false` depending on the Ruby code, and that could be used to optimize methods. It would return `true` if any subsequent code could be impacted by `$~` and al.

```ruby
def foo
  /x/ =~ 'x' # needs_last_match? # => false
  case method
  when /(foo)/ # needs_last_match? # => false
    do_something
  when /(bar)/ # needs_last_match? # => true
    puts $2
    # ... # needs_last_match? # => false
  end
end

def bar
  # ... # needs_last_match? # => true
  case x
  when /(foo)/ # needs_last_match? # => true
    do_something
  end
  Regexp.last_match
  # ... # needs_last_match? # => false (false negative)
  Regexp.send :last_match
  # ... # needs_last_match? # => false (false negative)
  const_get(:Regexp).last_match
end
```

----------------------------------------
Bug #17030: Enumerable#grep{_v} should be optimized for Regexp
https://bugs.ruby-lang.org/issues/17030#change-87199

* 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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