Is there an inverse 'member?' method in ruby?

You can easily define it along this line:

class Object
  def is_in? set
    set.include? self
  end
end

and then use as

8.is_in? [0, 9, 15]   # false
8.is_in? [0, 8, 15]   # true

or define

class Object
  def is_in? *set
    set.include? self
  end
end

and use as

8.is_in?(0, 9, 15)   # false
8.is_in?(0, 8, 15)   # true

In your specific case there's end_with?, which takes multiple arguments.

"Hello.".end_with?(',', '.') #=> true

Not in ruby but in ActiveSupport:

characters = ["Konata", "Kagami", "Tsukasa"]
"Konata".in?(characters) # => true

Not the answer for your question, but perhaps a solution for your problem.

word is a String, isn't it?

You may check with a regex:

end_index = word =~ /\A[\.,]/  ? -3 : -2

end_index = word.match(/\A[\.,]/)  ? -3 : -2

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