在Swift中， 要从HTML格式的String中截取出所有的img的所有图片url，要截取的url就要匹配url，需要用到正则表达式。

• Swift使用正则截取字符串的方法：

extension String {

    /** 根据 正则表达式 截取字符串 - parameter regex: 正则表达式 - returns: 字符串数组 */
    public func matchesForRegex(regex: String) -> [String]? {

        do {
            let regularExpression = try NSRegularExpression(pattern: regex,options: [])
            let range = NSMakeRange(0,self.characters.count)
            let results = regularExpression.matchesInString(self,options: [],range: range)
            let string = self as NSString
            return results.map { string.substringWithRange($0.range)}
        } catch {
            return nil
        }
    }
}

• 匹配img标签的正则表达式为：

<img\b(?=\s)(?=(?:[^>=]|='[^']*'|="[^"]*"|=[^'"][^\s>]*)*?\ssrc=['"]([^"]*)['"]?)(?:[^>=]|='[^']*'|="[^"]*"|=[^'"\s]*)*"\s?\/?>

• 在swift中的正则表达式书写为：

let regex = "<img\\b(?=\\s)(?=(?:[^>=]|='[^']*'|=\"[^\"]*\"|=[^'\"][^\\s>]*)*?\\ssrc=['\"]([^\"]*)['\"]?)(?:[^>=]|='[^']*'|=\"[^\"]*\"|=[^'\"\\s]*)*\"\\s?\\/?>"

• 使用案例：

let string = "图片1<div><img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\"><br></div><div>图片2</div><div><img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\"><br></div><div><br></div><div>图片3</div><div><img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\"><br></div><div><br></div>"

// 截取img标签
let resultItems = string.matchesForRegex(regex)

• 截取得到的resultItems为:

["<img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\">","<img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\">","<img src=\"http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg\" alt=\"Gravatar\">"]

• 如果单单只是截取img的URL地址，使用的正则表达式为：

(http[^\s]+(jpg|jpeg|png|tiff)\b)

• 在Swift中书写为：

let regex = "(http[^\\s]+(jpg|jpeg|png|tiff)\\b)"

• 使用案例：

// 截取所有img url
let resultItems = string.matchesForRegex(regex)

• 结果为：

[“http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg“,“http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg“,“http://cms-bucket.nosdn.127.net/catchpic/C/C7/C7643B52B98D04979AA173C6D6219958.jpg“]