How to decode an "=?UTF-8?B?…" string in Ruby

I am working with the Mandrill Inbound Email API, and when an email has an attachment with one or more spaces in its file name, then the file name is encoded in a format that I do not know how to decode.

Here is a an example string I receive for the file name: =?UTF-8?B?TWlzc2lvbmFyecKgRmFpdGjCoFByb21pc2XCoGFuZMKgQ2FzaMKgUmVjZWlwdHPCoFlURMKgMjUzNQ==?= =?UTF-8?B?OTnCoEp1bHktMjAxNS5jc3Y=?=

I tried Base64.decode64(#{encoded_value}) but that didn't return a readable text.

How do I decode that value into a readable string?

Solution

This is MIME encoded-word syntax as defined in RFC-2822. From Wikipedia:

The form is: "=?charset?encoding?encoded text?=".

charset may be any character set registered with IANA. Typically it would be the same charset as the message body.

encoding can be either "Q" denoting Q-encoding that is similar to the quoted-printable encoding, or "B" denoting base64 encoding.

encoded text is the Q-encoded or base64-encoded text.

Fortunately you don't need to write a decoder for this. The Mail gem comes with a Mail::Encodings.value_decode method that works perfectly and is very well-tested:

subject = "=?UTF-8?B?TWlzc2lvbmFyecKgRmFpdGjCoFByb21pc2XCoGFuZMKgQ2FzaMKgUmVjZWlwdHPCoFlURMKgMjUzNQ==?= =?UTF-8?B?OTnCoEp1bHktMjAxNS5jc3Y=?="
Mail::Encodings.value_decode(subject)
# => "Missionary Faith Promise and Cash Receipts YTD 253599 July-2015.csv"

It gracefully handles lots of edge cases you probably wouldn't think of (until your app tries to handle them and falls over):

subject = "Re:[=?iso-2022-jp?B?GyRCJTAlayE8JV0lcyEmJTglYyVRJXMzdDwwMnEbKEI=?=\n =?iso-2022-jp?B?GyRCPFIbKEI=?=] =?iso-2022-jp?B?GyRCSlY/LiEnGyhC?=\n  =?iso-2022-jp?B?GyRCIVolMCVrITwlXSVzIVskKkxkJCQ5ZyRvJDsbKEI=?=\n =?iso-2022-jp?B?GyRCJE43byRLJEQkJCRGIUolaiUvJSglOSVIGyhC?=#1056273\n =?iso-2022-jp?B?GyRCIUsbKEI=?="
Mail::Encodings.value_decode(subject)
# => "Re:[グルーポン・ジャパン株式会社] 返信：【グルーポン】お問い合わせの件について（リクエスト#1056273\n ）"

If you're using Rails you already have the Mail gem. Otherwise just add gem "mail" to your Gemfile, then bundle install and, in your script, require "mail".