switch to case and split

def caesar_cipher(str, shift = 5)
  alpha = ('a'..'z').to_a
  shifted = []

  str = str.split('')
  str.each do |e|
    shifted << case e
               when /[a-z]/
                 alpha[alpha.index(e) - shift]
               when /[A-Z]/
                 alpha[alpha.index(e)].upcase
               else
                 e
               end
  end

  shifted.join
end