Ruby: All about finding vowels

vowels

def is_vowel_elsif(letter)
  if letter == "a"
    true
  elsif letter == "e"
    true
  elsif letter == "i"
    true
  elsif letter == "o"
    true
  elsif letter == "u"
    true
  else
    false
  end
end

puts is_vowel_elsif("a")

def is_vowel_case(letter)
  case letter
  when "a"
    true
  when "e"
    true
  when "i"
    true
  when "o"
    true
  when "u"
    true
  else
    false
  end
end

puts is_vowel_case("e")

def is_vowel_one_if(letter)
  if letter == "a" || letter == "e" || letter == "i" || letter == "o" || letter == "u"
    return true
  end
  false
end

puts is_vowel_one_if("i")

def is_vowel_no_if(letter)
  "aeiou".include?(letter)
end

puts is_vowel_no_if("o")

def is_vowel_array(letter)
  ["a", "e", "i", "o", "u"].include?(letter)
end

puts is_vowel_array("u")

def first_vowel(string)
  string.match(/a|e|i|o|u/)
end

puts first_vowel("first_vowel")

def first_vowel_index(string)
  string.index(first_vowel(string).to_s)
end

puts first_vowel_index("first_vowel_index")