使用NSRegularExpression的奇怪的字符串范围行为匹配

Question

我试图解析一个原始的HTTP响应，并且在尝试将NSRange转换为Range时出现错误的范围。下面是一个游乐场的相关代码：

public extension NSRange { 
    public func toStringRange(_ str: String) -> Range<String.Index>? { 
     guard str.characters.count >= length - location && location < str.characters.count else { return nil } 
     let fromIdx = str.characters.index(str.startIndex, offsetBy: self.location) 
     print("from: \(self.location) = \(fromIdx)") 
     let toIdx = str.characters.index(fromIdx, offsetBy: self.length) 
     return fromIdx..<toIdx 
    } 
} 

let responseString = "HTTP/1.0 200 OK\r\nContent-Length: 193\r\nContent-Type: application/json\r\n" 
let responseRange = NSRange(location: 0, length: responseString.characters.count) 
let responseRegex = try! NSRegularExpression(pattern: "^(HTTP/1.\\d) (\\d+) (.*?\r\n)(.*)", options: [.anchorsMatchLines]) 
guard let matchResult = responseRegex.firstMatch(in: responseString, options: [], range: responseRange), 
    matchResult.numberOfRanges == 5, 
    let versionRange = matchResult.rangeAt(1).toStringRange(responseString), 
    let statusRange = matchResult.rangeAt(2).toStringRange(responseString), 
    let headersRange = matchResult.rangeAt(4).toStringRange(responseString) 
    else { fatalError() } 

从打印在toStringRange（输出）是

from: 0 = Index(_base: Swift.String.UnicodeScalarView.Index(_position: 0), _countUTF16: 1) 
from: 9 = Index(_base: Swift.String.UnicodeScalarView.Index(_position: 9), _countUTF16: 1) 
from: 17 = Index(_base: Swift.String.UnicodeScalarView.Index(_position: 18), _countUTF16: 1) 

为什么是第三toStringRange（）调用返回始于18，而不是字符串范围17？

Answer 1

你的转换方法从NSRange到Range<String.Index>不 工作正确地对外部的“基本多平面”（表情符号，标志，等） 扩展字形集群和字符。

NSRange计数UTF-16代码点（对应于unichar 表示在NSString）。 Range<String.Index>计数Swift Characters表示扩展字形集群。

在你的具体情况，"\r\n"算作两个UTF-16代码点，但 作为一个单一的Character，并引起不必要的“转变”。

下面是一个简单的例子：

let responseString = "OK\r\nContent-Length" 

let nsRange = (responseString as NSString).range(of: "Content") 
print(nsRange.location, nsRange.length) // 4 7 

if let sRange1 = nsRange.toStringRange(responseString) { 
    print(responseString.substring(with: sRange1)) // "ontent-" 
} 

从NSRange to Range<String.Index>使用方法

extension String { 
    func range(from nsRange: NSRange) -> Range<String.Index>? { 
     guard 
      let from16 = utf16.index(utf16.startIndex, offsetBy: nsRange.location, limitedBy: utf16.endIndex), 
      let to16 = utf16.index(from16, offsetBy: nsRange.length, limitedBy: utf16.endIndex), 
      let from = String.Index(from16, within: self), 
      let to = String.Index(to16, within: self) 
      else { return nil } 
     return from ..< to 
    } 
} 

你会得到预期的结果：

if let sRange2 = responseString.range(from: nsRange) { 
    print(responseString.substring(with: sRange2)) // "Content" 
}