Ruby | 正規表現の条件式に変数を使うと、名前付きキャプチャによる変数代入が出来ない ref: http://qiita.com/Yinaura/items/13bbc6b050b97139d0d2

file0.txt

/a (?<month>July)/ =~ 'Ruby is a July birth stone'

month # => "July"

file1.txt

a = 'a'

/#{a} (?<month>July)/ =~ 'Ruby is a July birth stone' # => 8

month #=> #<NameError: undefined local variable or method `month' for main:Object>

file2.txt

a = 'a'

'Ruby is a July birth stone'.match(/#{a} (?<month>July)/) { |matched| matched[:month] } # => July

file3.txt

a = 'a'

month = 'Ruby is a July birth stone'.match(/#{a} (?<month>July)/) { |matched| matched[:month] } 

month # => "July"

file4.txt

- Ruby 2.2.4