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文字列の部分一致とデフォルト値のイディオム

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【論理演算子】||のデフォルト値を与えるイディオム の変化形です。
ある文字列に正規表現がマッチするなら一致部分を、そうでないならデフォルト値を与えます。

""[/[A-Za-z]+/] || "default"
#=> "default"
"ATOZ"[/[A-Za-z]+/] || "default"
#=>"ATOZ"
"AtoZ"[/[A-Za-z]+/] || "default"
#=> "AtoZ"
"009"[/[A-Za-z]+/] || "default"
#=> "default"

これはStringオブジェクトに[]を与えると、一致部分もしくは不一致の場合にnilが返却されることを利用しています。[]の引数にはRegexpだけでなく範囲指定や文字列指定も可能です。
instance method String#[]

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