ri ってメソッドも引けたのか

---------------------------------------------------------- Class: String
     A +String+ object holds and manipulates an arbitrary sequence of
     bytes, typically representing characters. String objects may be
     created using +String::new+ or as literals.

 ...

$ ri String.sub

------------------------------------------------------------- String#sub
     str.sub(pattern, replacement)         => new_str
     str.sub(pattern) {|match| block }     => new_str
------------------------------------------------------------------------

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