全角文字を半角文字に変換する

全角文字を使いこなす人頭良すぎる。
どうやって入力しているのか想像できない。

function zen2han(byval text)
    dim i, buf, ch
    i = 1
    buf = ""
    do while i <= len(text)
        ch = mid(text, i, 1)
        select case ch
        case "／" ch = "/" ' 意図的に使用される全角文字
        case "．" ch = "." ' 半角記号に見えて実は全角というパターン
        case "，" ch = "," ' 半角記号に見えて実は全角というパターン
        case "－" ch = "-" ' 半角記号に見えて実は全角というパターン
        case "―" ch = "-" ' 半角記号に見えて実は全角というパターン
        case "‐" ch = "-" ' 半角記号に見えて実は全角というパターン
        case "♯" ch = "#" ' 信じられないことに音楽記号を使ってくる人が居ます
        case "　" ch = " "
        case "！" ch = "!"
        case "”" ch = """"
        case "＃" ch = "#"
        case "＄" ch = "$"
        case "％" ch = "%"
        case "＆" ch = "&"
        case "’" ch = "'"
        case "（" ch = "("
        case "）" ch = ")"
        case "＝" ch = "="
        case "ー" ch = "-"
        case "～" ch = "~"
        case "＾" ch = "^"
        case "｜" ch = "|"
        case "￥" ch = "\"
        case "「" ch = "["
        case "」" ch = "]"
        case "｛" ch = "{"
        case "｝" ch = "}"
        case "｀" ch = "`"
        case "＠" ch = "@"
        case "＊" ch = "*"
        case "：" ch = ":"
        case "；" ch = ";"
        case "＋" ch = "+"
        case "＜" ch = "<"
        case "＞" ch = ">"
        case "、" ch = ","
        case "。" ch = "."
        case "？" ch = "?"
        case "・" ch = "/"
        case "＿" ch = "_"
        case "１" ch = "1"
        case "２" ch = "2"
        case "３" ch = "3"
        case "４" ch = "4"
        case "５" ch = "5"
        case "６" ch = "6"
        case "７" ch = "7"
        case "８" ch = "8"
        case "９" ch = "9"
        case "０" ch = "0"
        case "ａ" ch = "a"
        case "ｂ" ch = "b"
        case "ｃ" ch = "c"
        case "ｄ" ch = "d"
        case "ｅ" ch = "e"
        case "ｆ" ch = "f"
        case "ｇ" ch = "g"
        case "ｈ" ch = "h"
        case "ｉ" ch = "i"
        case "ｊ" ch = "j"
        case "ｋ" ch = "k"
        case "ｌ" ch = "l"
        case "ｍ" ch = "m"
        case "ｎ" ch = "n"
        case "ｏ" ch = "o"
        case "ｐ" ch = "p"
        case "ｑ" ch = "q"
        case "ｒ" ch = "r"
        case "ｓ" ch = "s"
        case "ｔ" ch = "t"
        case "ｕ" ch = "u"
        case "ｖ" ch = "v"
        case "ｗ" ch = "w"
        case "ｘ" ch = "x"
        case "ｙ" ch = "y"
        case "ｚ" ch = "z"
        case "Ａ" ch = "A"
        case "Ｂ" ch = "B"
        case "Ｃ" ch = "C"
        case "Ｄ" ch = "D"
        case "Ｅ" ch = "E"
        case "Ｆ" ch = "F"
        case "Ｇ" ch = "G"
        case "Ｈ" ch = "H"
        case "Ｉ" ch = "I"
        case "Ｊ" ch = "J"
        case "Ｋ" ch = "K"
        case "Ｌ" ch = "L"
        case "Ｍ" ch = "M"
        case "Ｎ" ch = "N"
        case "Ｏ" ch = "O"
        case "Ｐ" ch = "P"
        case "Ｑ" ch = "Q"
        case "Ｒ" ch = "R"
        case "Ｓ" ch = "S"
        case "Ｔ" ch = "T"
        case "Ｕ" ch = "U"
        case "Ｖ" ch = "V"
        case "Ｗ" ch = "W"
        case "Ｘ" ch = "X"
        case "Ｙ" ch = "Y"
        case "Ｚ" ch = "Z"
        end select
        buf = buf & ch
        i = i + 1
    loop
    zen2han = buf
end function