解码 Swift 中引用的可打印消息

Question

我有一个带引号的可打印字符串，例如 "The cost would be =C2=A31,000"。我如何将其转换为 "The cost would be £1,000".

我目前只是手动转换文本，这并不涵盖所有情况。我确信只有一行代码可以帮助解决这个问题。

这是我的代码：

func decodeUTF8(message: String) -> String
{
    var newMessage = message.stringByReplacingOccurrencesOfString("=2E", withString: ".", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=E2=80=A2", withString: "•", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=C2=A3", withString: "£", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=A3", withString: "£", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=E2=80=9C", withString: "\"", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=E2=80=A6", withString: "…", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=E2=80=9D", withString: "\"", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=92", withString: "'", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=3D", withString: "=", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=20", withString: "", options: NSStringCompareOptions.LiteralSearch, range: nil)
    newMessage = newMessage.stringByReplacingOccurrencesOfString("=E2=80=99", withString: "'", options: NSStringCompareOptions.LiteralSearch, range: nil)

    return newMessage
}

谢谢

Answer 1

这种编码称为 'quoted-printable'，您需要做的是使用 ASCII 编码将字符串转换为 NSData，然后迭代数据，用 byte/char 0xA3，然后使用NSUTF8StringEncoding.

将结果数据转换为字符串

Answer 2

为了给出适用的解决方案，还需要一些信息。所以，我会做一些假设。

例如，在 HTML 或邮件消息中，您可以将一种或多种编码应用于某种源数据。例如，您可以对二进制文件进行编码，例如一个带有 base64 的 png 文件，然后将其压缩。顺序很重要。

在您所说的示例中，源数据是一个字符串并且已通过 UTF-8 编码。

在 HTTP 消息中，您的 Content-Type 因此是 text/plain; charset = UTF-8。在您的示例中，似乎还应用了额外的编码， a "Content-Transfer-Encoding"：可能 Content-transfer-encoding 是 quoted-printable 或 base64（虽然不确定）。

为了还原它，您需要以相反的顺序应用相应的解码。

提示:

您可以在查看邮件的原始来源时查看邮件的 headers（Contente-type 和 Content-Transfer-Encoding）。

Answer 3

一个简单的方法是使用 (NS)String 方法 stringByRemovingPercentEncoding 为此目的。这是观察到的 decoding quoted-printables, 所以第一个解决方案主要是对答案的翻译该线程 Swift.

想法是用 quoted-printable "=NN" 编码替换 percent encoding "%NN" 然后用现有的方法去掉百分比编码。

续行单独处理。此外，输入字符串中的百分比字符必须首先 encoded，否则他们将被视为百分比中的主角编码。

func decodeQuotedPrintable(message : String) -> String? {
    return message
        .stringByReplacingOccurrencesOfString("=\r\n", withString: "")
        .stringByReplacingOccurrencesOfString("=\n", withString: "")
        .stringByReplacingOccurrencesOfString("%", withString: "%25")
        .stringByReplacingOccurrencesOfString("=", withString: "%")
        .stringByRemovingPercentEncoding
}

函数 returns 一个可选字符串，它是 nil 表示无效输入。无效输入可以是：

一个“=”字符，后面没有跟两个十六进制数字，例如"=XX".
未解码为有效 UTF-8 序列的“=NN”序列，例如"=E2=64".

示例：

if let decoded = decodeQuotedPrintable("=C2=A31,000") {
    print(decoded) // £1,000
}

if let decoded = decodeQuotedPrintable("=E2=80=9CHello =E2=80=A6 world!=E2=80=9D") {
    print(decoded) // “Hello … world!”
}

更新1：以上代码假设消息使用UTF-8 用于引用 non-ASCII 个字符的编码，如大多数示例所示：C2 A3 是“£”的 UTF-8 编码，E2 80 A4 是 … 的 UTF-8 编码.

如果输入是 "Rub=E9n" 则消息使用 Windows-1252编码。要正确解码，您必须替换

.stringByRemovingPercentEncoding

来自

.stringByReplacingPercentEscapesUsingEncoding(NSWindowsCP1252StringEncoding)

还有一些方法可以从 "Content-Type" 中检测编码 header 字段，比较例如https://whosebug.com/a/32051684/1187415.

更新二： stringByReplacingPercentEscapesUsingEncoding 方法被标记为已弃用，因此上面的代码将始终生成编译器警告。不幸的是，似乎没有替代方法已由 Apple 提供。

所以这里有一个新的、完全 self-contained 的解码方法不会引起任何编译器警告。这次我写了作为 String 的扩展方法。解释评论在代码。

extension String {

    /// Returns a new string made by removing in the `String` all "soft line
    /// breaks" and replacing all quoted-printable escape sequences with the
    /// matching characters as determined by a given encoding. 
    /// - parameter encoding:     A string encoding. The default is UTF-8.
    /// - returns:                The decoded string, or `nil` for invalid input.

    func decodeQuotedPrintable(encoding enc : NSStringEncoding = NSUTF8StringEncoding) -> String? {

        // Handle soft line breaks, then replace quoted-printable escape sequences. 
        return self
            .stringByReplacingOccurrencesOfString("=\r\n", withString: "")
            .stringByReplacingOccurrencesOfString("=\n", withString: "")
            .decodeQuotedPrintableSequences(enc)
    }

    /// Helper function doing the real work.
    /// Decode all "=HH" sequences with respect to the given encoding.

    private func decodeQuotedPrintableSequences(enc : NSStringEncoding) -> String? {

        var result = ""
        var position = startIndex

        // Find the next "=" and copy characters preceding it to the result:
        while let range = rangeOfString("=", range: position ..< endIndex) {
            result.appendContentsOf(self[position ..< range.startIndex])
            position = range.startIndex

            // Decode one or more successive "=HH" sequences to a byte array:
            let bytes = NSMutableData()
            repeat {
                let hexCode = self[position.advancedBy(1) ..< position.advancedBy(3, limit: endIndex)]
                if hexCode.characters.count < 2 {
                    return nil // Incomplete hex code
                }
                guard var byte = UInt8(hexCode, radix: 16) else {
                    return nil // Invalid hex code
                }
                bytes.appendBytes(&byte, length: 1)
                position = position.advancedBy(3)
            } while position != endIndex && self[position] == "="

            // Convert the byte array to a string, and append it to the result:
            guard let dec = String(data: bytes, encoding: enc) else {
                return nil // Decoded bytes not valid in the given encoding
            }
            result.appendContentsOf(dec)
        }

        // Copy remaining characters to the result:
        result.appendContentsOf(self[position ..< endIndex])

        return result
    }
}

用法示例：

if let decoded = "=C2=A31,000".decodeQuotedPrintable() {
    print(decoded) // £1,000
}

if let decoded = "=E2=80=9CHello =E2=80=A6 world!=E2=80=9D".decodeQuotedPrintable() {
    print(decoded) // “Hello … world!”
}

if let decoded = "Rub=E9n".decodeQuotedPrintable(encoding: NSWindowsCP1252StringEncoding) {
    print(decoded) // Rubén
}

Swift4（及更高版本）的更新：

extension String {

    /// Returns a new string made by removing in the `String` all "soft line
    /// breaks" and replacing all quoted-printable escape sequences with the
    /// matching characters as determined by a given encoding.
    /// - parameter encoding:     A string encoding. The default is UTF-8.
    /// - returns:                The decoded string, or `nil` for invalid input.

    func decodeQuotedPrintable(encoding enc : String.Encoding = .utf8) -> String? {

        // Handle soft line breaks, then replace quoted-printable escape sequences.
        return self
            .replacingOccurrences(of: "=\r\n", with: "")
            .replacingOccurrences(of: "=\n", with: "")
            .decodeQuotedPrintableSequences(encoding: enc)
    }

    /// Helper function doing the real work.
    /// Decode all "=HH" sequences with respect to the given encoding.

    private func decodeQuotedPrintableSequences(encoding enc : String.Encoding) -> String? {

        var result = ""
        var position = startIndex

        // Find the next "=" and copy characters preceding it to the result:
        while let range = range(of: "=", range: position..<endIndex) {
            result.append(contentsOf: self[position ..< range.lowerBound])
            position = range.lowerBound

            // Decode one or more successive "=HH" sequences to a byte array:
            var bytes = Data()
            repeat {
                let hexCode = self[position...].dropFirst().prefix(2)
                if hexCode.count < 2 {
                    return nil // Incomplete hex code
                }
                guard let byte = UInt8(hexCode, radix: 16) else {
                    return nil // Invalid hex code
                }
                bytes.append(byte)
                position = index(position, offsetBy: 3)
            } while position != endIndex && self[position] == "="

            // Convert the byte array to a string, and append it to the result:
            guard let dec = String(data: bytes, encoding: enc) else {
                return nil // Decoded bytes not valid in the given encoding
            }
            result.append(contentsOf: dec)
        }

        // Copy remaining characters to the result:
        result.append(contentsOf: self[position ..< endIndex])

        return result
    }
}

用法示例：

if let decoded = "=C2=A31,000".decodeQuotedPrintable() {
    print(decoded) // £1,000
}

if let decoded = "=E2=80=9CHello =E2=80=A6 world!=E2=80=9D".decodeQuotedPrintable() {
    print(decoded) // “Hello … world!”
}

if let decoded = "Rub=E9n".decodeQuotedPrintable(encoding: .windowsCP1252) {
    print(decoded) // Rubén
}

Answer 4

不幸的是，我的回答有点晚了。不过，这可能对其他人有所帮助。

var string = "The cost would be =C2=A31,000"

var finalString: String? = nil

if let regEx = try? NSRegularExpression(pattern: "={1}?([a-f0-9]{2}?)", options: NSRegularExpressionOptions.CaseInsensitive)
{
    let intermediatePercentEscapedString = regEx.stringByReplacingMatchesInString(string, options: NSMatchingOptions.WithTransparentBounds, range: NSMakeRange(0, string.characters.count), withTemplate: "%")
    print(intermediatePercentEscapedString)
    finalString = intermediatePercentEscapedString.stringByRemovingPercentEncoding
    print(finalString)
}

Answer 5

您也可以查看这个可行的解决方案 - https://github.com/dunkelstern/QuotedPrintable

let result = QuotedPrintable.decode(string: quoted)

解码 Swift 中引用的可打印消息

Decoding quoted-printable messages in Swift

macos

utf-8

quoted-printable

swift