使用Swift Regex Builder按名称访问可选捕获组的方法

问题场景 #

输入行仅两种格式：要么是单个姓名单词，要么是以Test 开头后接姓名。需要提取姓名，同时无需通过匹配索引，安全访问Test 的匹配结果（不存在时为nil）。当前代码在匹配无Test 的行时会崩溃，原因是未匹配到的可选捕获组通过引用访问时发生类型转换错误。

原代码示例 #

import RegexBuilder

let line1 = "Test John"
let line2 = "Robert"

let nameReference = Reference(String.self)
let testReference = Reference(String.self)

let regex = Regex {
    Optionally {
        Capture(as:testReference) {
            "Test "
        } transform : { text in
            String(text)
        }
    }
    Capture(as:nameReference) {
        OneOrMore(.any)
    } transform : { text in
        String(text)
    }
}

if let matches = try? regex.wholeMatch(in: line1) { // 可切换为line2测试
    let theName = matches[nameReference]
    print("Name is \(theName)")
    // 通过索引访问正常
    if let flag = matches.1, flag == "Test " {
        print("Using index: This is a test line")
    } else {
        print("Using index: Not a test line")
    }
    // 通过引用访问在line2时崩溃
    if matches[testReference] == "Test " { 
        print("Using reference: This is a test line")
    } else {
        print("Using reference: Not a test line")
    }
}

运行结果 #

• 匹配line1时输出正常：

Name is John
Using index: This is a test line
Using reference: This is a test line

• 匹配line2时崩溃，输出：

Name is Robert
Using index: Not a test line
Could not cast value of type 'Swift.Optional<Swift.Substring>' (0x7ff84bf06f20) to 'Swift.String' (0x7ff84ba6e918).

解决方案 #

问题核心：可选捕获组的匹配结果本质是可选类型，但原代码声明Reference(String.self)未体现可选性，导致未匹配时强制转换失败。以下两种方式可解决：

方式一：声明可选类型的Reference #

将testReference声明为Reference(String?.self)，让未匹配时返回nil而非崩溃：

import RegexBuilder

let line1 = "Test John"
let line2 = "Robert"

let nameReference = Reference(String.self)
// 声明为可选String类型的引用
let testReference = Reference(String?.self)

let regex = Regex {
    Optionally {
        Capture(as: testReference) {
            "Test "
        } transform: { text in
            String(text)
        }
    }
    Capture(as: nameReference) {
        OneOrMore(.any)
    } transform: { text in
        String(text)
    }
}

if let matches = try? regex.wholeMatch(in: line2) {
    let theName = matches[nameReference]
    print("Name is \(theName)")
    
    // 安全访问testReference，无崩溃
    if matches[testReference] == "Test " {
        print("Using reference: This is a test line")
    } else {
        print("Using reference: Not a test line")
    }
}

方式二：使用命名捕获组结合AnyRegexOutput #

利用AnyRegexOutput的下标特性，通过捕获组名称访问，未匹配时返回nil：

import RegexBuilder

let line1 = "Test John"
let line2 = "Robert"

let regex = Regex {
    Optionally {
        Capture(named: "testFlag") {
            "Test "
        } transform: { text in
            String(text)
        }
    }
    Capture(named: "name") {
        OneOrMore(.any)
    } transform: { text in
        String(text)
    }
}

if let matches = try? regex.wholeMatch(in: line2) as AnyRegexOutput {
    let theName = matches["name"] as! String
    print("Name is \(theName)")
    
    // 通过名称安全访问，未匹配时返回nil
    if matches["testFlag"] as? String == "Test " {
        print("Using named capture: This is a test line")
    } else {
        print("Using named capture: Not a test line")
    }
}

关键说明 #

• 当捕获组被包裹在Optionally内部时，匹配结果为可选值，Reference类型需对应Optional<T>才能安全访问。
• AnyRegexOutput的下标支持通过捕获组名称访问，未匹配时返回nil，适合不想使用Reference的场景。

内容的提问来源于stack exchange，提问作者rene