How do I decode HTML entities in Swift?

Question

I am pulling a JSON file from a site and one of the strings received is:

The Weeknd ‘King Of The Fall&         


        

Answer 1

@akashivskyy's answer is great and demonstrates how to utilize NSAttributedString to decode HTML entities. One possible disadvantage (as he stated) is that all HTML markup is removed as well, so

 4 < 5 & 3 > 2

becomes

4 < 5 & 3 > 2

On OS X there is CFXMLCreateStringByUnescapingEntities() which does the job:

let encoded = " 4 < 5 & 3 > 2 . Price: 12 €.  @ "
let decoded = CFXMLCreateStringByUnescapingEntities(nil, encoded, nil) as String
println(decoded)
//  4 < 5 & 3 > 2 . Price: 12 €.  @ 

but this is not available on iOS.

Here is a pure Swift implementation. It decodes character entities references like < using a dictionary, and all numeric character entities like @ or €. (Note that I did not list all 252 HTML entities explicitly.)

Swift 4:

// Mapping from XML/HTML character entity reference to character
// From http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references
private let characterEntities : [ Substring : Character ] = [
    // XML predefined entities:
    """    : "\"",
    "&"     : "&",
    "'"    : "'",
    "<"      : "<",
    ">"      : ">",

    // HTML character entity references:
    " "    : "\u{00a0}",
    // ...
    "♦"   : "♦",
]

extension String {

    /// Returns a new string made by replacing in the `String`
    /// all HTML character entity references with the corresponding
    /// character.
    var stringByDecodingHTMLEntities : String {

        // ===== Utility functions =====

        // Convert the number in the string to the corresponding
        // Unicode character, e.g.
        //    decodeNumeric("64", 10)   --> "@"
        //    decodeNumeric("20ac", 16) --> "€"
        func decodeNumeric(_ string : Substring, base : Int) -> Character? {
            guard let code = UInt32(string, radix: base),
                let uniScalar = UnicodeScalar(code) else { return nil }
            return Character(uniScalar)
        }

        // Decode the HTML character entity to the corresponding
        // Unicode character, return `nil` for invalid input.
        //     decode("@")    --> "@"
        //     decode("€") --> "€"
        //     decode("<")     --> "<"
        //     decode("&foo;")    --> nil
        func decode(_ entity : Substring) -> Character? {

            if entity.hasPrefix("&#x") || entity.hasPrefix("&#X") {
                return decodeNumeric(entity.dropFirst(3).dropLast(), base: 16)
            } else if entity.hasPrefix("&#") {
                return decodeNumeric(entity.dropFirst(2).dropLast(), base: 10)
            } else {
                return characterEntities[entity]
            }
        }

        // ===== Method starts here =====

        var result = ""
        var position = startIndex

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

            // Find the next ';' and copy everything from '&' to ';' into `entity`
            guard let semiRange = self[position...].range(of: ";") else {
                // No matching ';'.
                break
            }
            let entity = self[position ..< semiRange.upperBound]
            position = semiRange.upperBound

            if let decoded = decode(entity) {
                // Replace by decoded character:
                result.append(decoded)
            } else {
                // Invalid entity, copy verbatim:
                result.append(contentsOf: entity)
            }
        }
        // Copy remaining characters to `result`:
        result.append(contentsOf: self[position...])
        return result
    }
}

Example:

let encoded = " 4 < 5 & 3 > 2 . Price: 12 €.  @ "
let decoded = encoded.stringByDecodingHTMLEntities
print(decoded)
//  4 < 5 & 3 > 2 . Price: 12 €.  @

---

Swift 3:

// Mapping from XML/HTML character entity reference to character
// From http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references
private let characterEntities : [ String : Character ] = [
    // XML predefined entities:
    """    : "\"",
    "&"     : "&",
    "'"    : "'",
    "<"      : "<",
    ">"      : ">",

    // HTML character entity references:
    " "    : "\u{00a0}",
    // ...
    "♦"   : "♦",
]

extension String {

    /// Returns a new string made by replacing in the `String`
    /// all HTML character entity references with the corresponding
    /// character.
    var stringByDecodingHTMLEntities : String {

        // ===== Utility functions =====

        // Convert the number in the string to the corresponding
        // Unicode character, e.g.
        //    decodeNumeric("64", 10)   --> "@"
        //    decodeNumeric("20ac", 16) --> "€"
        func decodeNumeric(_ string : String, base : Int) -> Character? {
            guard let code = UInt32(string, radix: base),
                let uniScalar = UnicodeScalar(code) else { return nil }
            return Character(uniScalar)
        }

        // Decode the HTML character entity to the corresponding
        // Unicode character, return `nil` for invalid input.
        //     decode("@")    --> "@"
        //     decode("€") --> "€"
        //     decode("<")     --> "<"
        //     decode("&foo;")    --> nil
        func decode(_ entity : String) -> Character? {

            if entity.hasPrefix("&#x") || entity.hasPrefix("&#X"){
                return decodeNumeric(entity.substring(with: entity.index(entity.startIndex, offsetBy: 3) ..< entity.index(entity.endIndex, offsetBy: -1)), base: 16)
            } else if entity.hasPrefix("&#") {
                return decodeNumeric(entity.substring(with: entity.index(entity.startIndex, offsetBy: 2) ..< entity.index(entity.endIndex, offsetBy: -1)), base: 10)
            } else {
                return characterEntities[entity]
            }
        }

        // ===== Method starts here =====

        var result = ""
        var position = startIndex

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

            // Find the next ';' and copy everything from '&' to ';' into `entity`
            if let semiRange = self.range(of: ";", range: position ..< endIndex) {
                let entity = self[position ..< semiRange.upperBound]
                position = semiRange.upperBound

                if let decoded = decode(entity) {
                    // Replace by decoded character:
                    result.append(decoded)
                } else {
                    // Invalid entity, copy verbatim:
                    result.append(entity)
                }
            } else {
                // No matching ';'.
                break
            }
        }
        // Copy remaining characters to `result`:
        result.append(self[position ..< endIndex])
        return result
    }
}

---

Swift 2:

// Mapping from XML/HTML character entity reference to character
// From http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references
private let characterEntities : [ String : Character ] = [
    // XML predefined entities:
    """    : "\"",
    "&"     : "&",
    "'"    : "'",
    "<"      : "<",
    ">"      : ">",

    // HTML character entity references:
    " "    : "\u{00a0}",
    // ...
    "♦"   : "♦",
]

extension String {

    /// Returns a new string made by replacing in the `String`
    /// all HTML character entity references with the corresponding
    /// character.
    var stringByDecodingHTMLEntities : String {

        // ===== Utility functions =====

        // Convert the number in the string to the corresponding
        // Unicode character, e.g.
        //    decodeNumeric("64", 10)   --> "@"
        //    decodeNumeric("20ac", 16) --> "€"
        func decodeNumeric(string : String, base : Int32) -> Character? {
            let code = UInt32(strtoul(string, nil, base))
            return Character(UnicodeScalar(code))
        }

        // Decode the HTML character entity to the corresponding
        // Unicode character, return `nil` for invalid input.
        //     decode("@")    --> "@"
        //     decode("€") --> "€"
        //     decode("<")     --> "<"
        //     decode("&foo;")    --> nil
        func decode(entity : String) -> Character? {

            if entity.hasPrefix("&#x") || entity.hasPrefix("&#X"){
                return decodeNumeric(entity.substringFromIndex(entity.startIndex.advancedBy(3)), base: 16)
            } else if entity.hasPrefix("&#") {
                return decodeNumeric(entity.substringFromIndex(entity.startIndex.advancedBy(2)), base: 10)
            } else {
                return characterEntities[entity]
            }
        }

        // ===== Method starts here =====

        var result = ""
        var position = startIndex

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

            // Find the next ';' and copy everything from '&' to ';' into `entity`
            if let semiRange = self.rangeOfString(";", range: position ..< endIndex) {
                let entity = self[position ..< semiRange.endIndex]
                position = semiRange.endIndex

                if let decoded = decode(entity) {
                    // Replace by decoded character:
                    result.append(decoded)
                } else {
                    // Invalid entity, copy verbatim:
                    result.appendContentsOf(entity)
                }
            } else {
                // No matching ';'.
                break
            }
        }
        // Copy remaining characters to `result`:
        result.appendContentsOf(self[position ..< endIndex])
        return result
    }
}