[ruby-core:99228] [Ruby master Feature#17036] Regexp deconstruction keys to allow pattern matching

[cottrey@zendesk.com]

Issue #17036 has been reported by zechris (Chris Ottrey).

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Feature #17036: Regexp deconstruction keys to allow pattern matching
https://bugs.ruby-lang.org/issues/17036

* Author: zechris (Chris Ottrey)
* Status: Open
* Priority: Normal
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This is to allow Regexp matches in Ruby 2.7's new experimental Pattern Matching.

eg.
```ruby
  case /(?<a>.)(?<b>.)/.match("xy")
  in a: "x", b:
    "a was 'x' and b was matched to #{b.inspect}"
  end

  #=> "a was 'x' and b was matched to 'y'"
```

`MatchData#named_captures` is *close* but not close enough as the Hash required from a `deconstructed_keys()` method that gets used during Pattern Matching requires symbolized keys.

### **NB. This PR ( https://github.com/ruby/ruby/pull/3333) is WIP as the C code hasn't actually been written yet...**

Although, it could be used in the wild now with a refinement of `MatchData` like so:
```ruby
module RegexpDeconstructKeys
  refine MatchData do
    # Temporary patch in ruby to make tests pass until the code gets re-written in C
    def deconstruct_keys(*keys)
      h0 = named_captures.transform_keys(&:to_sym)
      if keys
        keys = keys.flatten.compact
        raise TypeError unless keys.all? { |k| k.is_a?(Symbol) }
        h0 = h0.slice(*keys) unless keys.empty?
      end
      h0.inject({}) { |h, (k, v)| h[k] = v if v; h }
    end
    # Temporary patch in ruby to make tests pass until the code gets re-written in C
  end
end

using RegexpDeconstructKeys
case /(?<a>.)(?<b>.)/.match("xy")
in a: "x", b:
  puts "a was 'x' and b was #{b.inspect}"
end

#=> "a was 'x' and b was matched to 'y'"
```
(tested in `ruby v2.7.1`)




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