macos - 在 Swift 中解码引用的可打印消息

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“费用为 1,000 英镑”。

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

一个简单的方法是使用 (NS)String 方法stringByRemovingPercentEncoding 用于此目的。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到 Swift。

想法是将引用打印的“=NN”编码替换为percent encoding "%NN"然后用现有的方法去掉百分比编码。

续行是单独处理的。此外，输入字符串中的百分比字符必须首先编码，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
}

该函数返回一个可选字符串，对于无效输入，该字符串为 nil。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”。
• 不能解码为有效 UTF-8 序列的“=NN”序列，例如“=E2=64”。

例子:

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:以上代码假定消息使用 UTF-8用于引用非 ASCII 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 UTF-8 … 的编码。

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

.stringByRemovingPercentEncoding

通过

.stringByReplacingPercentEscapesUsingEncoding(NSWindowsCP1252StringEncoding)

还有一些方法可以从“Content-Type”中检测编码标题字段，比较例如https://stackoverflow.com/a/32051684/1187415 .

---

更新 2: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，似乎没有替代方法由 Apple 提供。

所以这是一种新的、完全独立的解码方法，它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
    }
}

示例用法:

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
}

---

Swift 4(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
    }
}

示例用法:

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
}