[ruby-core:105290] [Ruby master Feature#18172] MatchData#sublen to return the length of a substring

Issue #18172 has been reported by nobu (Nobuyoshi Nakada).

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Feature #18172: MatchData#sublen to return the length of a substring
https://bugs.ruby-lang.org/issues/18172

* Author: nobu (Nobuyoshi Nakada)
* Status: Open
* Priority: Normal
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There are many code taking the length of a substring matched by `Regexp`.
For instance, in rdoc/markup/attribute_manager.rb:
```ruby
      attr_updated = attrs.set_attrs($`.length + $1.length + $2.length, $3.length, attr)
      if attr_updated
        $1 + NULL * $2.length + $3 + NULL * $2.length + $4
```

People often tends to use such code (although the first addition can be simpler as `$~.begin(3)`), that creates and soon drops substrings, just to take the length.

Therefore, how about the new method to calculate the length, `MatchData#sublen`?
```ruby
/(\d+)\W(\w+)/ =~ "1:foo"
$~.sublen(1)    #=> 1
$~.sublen(2)    #=> 3
```




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