濁点(\u3099)と半濁点(\u309A)を別扱いしている文字を結合する

bind_dakuten.rb

DAKUTEN = "\u3099".freeze
HANDAKUTEN = "\u309A".freeze

BIND_LIST = {
  "か#{DAKUTEN}" => "が",
  "き#{DAKUTEN}" => "ぎ",
  "く#{DAKUTEN}" => "ぐ",
  "け#{DAKUTEN}" => "げ",
  "こ#{DAKUTEN}" => "ご",
  "さ#{DAKUTEN}" => "ざ",
  "し#{DAKUTEN}" => "じ",
  "す#{DAKUTEN}" => "ず",
  "せ#{DAKUTEN}" => "ぜ",
  "そ#{DAKUTEN}" => "ぞ",
  "た#{DAKUTEN}" => "だ",
  "ち#{DAKUTEN}" => "ぢ",
  "つ#{DAKUTEN}" => "づ",
  "て#{DAKUTEN}" => "で",
  "と#{DAKUTEN}" => "ど",
  "は#{DAKUTEN}" => "ば",
  "は#{HANDAKUTEN}" => "ぱ",
  "ひ#{DAKUTEN}" => "び",
  "ひ#{HANDAKUTEN}" => "ぴ",
  "ふ#{DAKUTEN}" => "ぶ",
  "ふ#{HANDAKUTEN}" => "ぷ",
  "へ#{DAKUTEN}" => "べ",
  "へ#{HANDAKUTEN}" => "ぺ",
  "ほ#{DAKUTEN}" => "ぼ",
  "ほ#{HANDAKUTEN}" => "ぽ",
  "カ#{DAKUTEN}" => "ガ",
  "キ#{DAKUTEN}" => "ギ",
  "ク#{DAKUTEN}" => "グ",
  "ケ#{DAKUTEN}" => "ゲ",
  "コ#{DAKUTEN}" => "ゴ",
  "サ#{DAKUTEN}" => "ザ",
  "シ#{DAKUTEN}" => "ジ",
  "ス#{DAKUTEN}" => "ズ",
  "セ#{DAKUTEN}" => "ゼ",
  "ソ#{DAKUTEN}" => "ゾ",
  "タ#{DAKUTEN}" => "ダ",
  "チ#{DAKUTEN}" => "ヂ",
  "ツ#{DAKUTEN}" => "ヅ",
  "テ#{DAKUTEN}" => "デ",
  "ト#{DAKUTEN}" => "ド",
  "ハ#{DAKUTEN}" => "バ",
  "ハ#{HANDAKUTEN}" => "パ",
  "ヒ#{DAKUTEN}" => "ビ",
  "ヒ#{HANDAKUTEN}" => "ピ",
  "フ#{DAKUTEN}" => "ブ",
  "フ#{HANDAKUTEN}" => "プ",
  "ヘ#{DAKUTEN}" => "ベ",
  "ヘ#{HANDAKUTEN}" => "ペ",
  "ホ#{DAKUTEN}" => "ボ",
  "ホ#{HANDAKUTEN}" => "ポ"
}.freeze

def bind_dakuten(str)
  result = str.dup
  BIND_LIST.each do |k, v|
    result.gsub!(k, v)
  end
  result
end

trを使うとこんなふうにできます。
def bind_dakuten(str)
str.gsub(/(.)(\u3099|\u309B)/){|w|
$1.tr('か-とは-ほカ-トハ-ホウ','が-どば-ぼガ-ドバ-ボヴ')
}.gsub(/(.)(\u309A|\u309C)/){|w|
$1.tr('は-ほハ-ホ','ぱ-ぽパ-ポ')
}
end