是否可以编写一个 Swift 函数来仅替换扩展字素簇的一部分，例如

我想编写一个可以像这样使用的函数：

let ????‍????‍????‍???? = "????‍????‍????‍????".replacingFirstOccurrence(of: "????", with: "????")

Given 这个字符串和 Swift 的字符串都很奇怪String图书馆是 https://stackoverflow.com/questions/43618487/why-is-treated-so-strangely-in-swift-strings/43619038#43619038，这在 Swift 中可能吗？

基于获得的见解为什么像 ????‍????‍????‍???? 这样的表情符号在 Swift 字符串中的处理方式如此奇怪？ https://stackoverflow.com/questions/43618487/why-is-treated-so-strangely-in-swift-strings，一个明智的方法可能是替换 Unicode 标量：

extension String {
    func replacingFirstOccurrence(of target: UnicodeScalar, with replacement: UnicodeScalar) -> String {

        let uc = self.unicodeScalars
        guard let idx = uc.index(of: target) else { return self }
        let prefix = uc[uc.startIndex..<idx]
        let suffix = uc[uc.index(after: idx) ..< uc.endIndex]
        return "\(prefix)\(replacement)\(suffix)"
    }
}

Example:

let family1 = "????‍????‍????‍????"
print(family1.characters.map { Array(String($0).unicodeScalars) })
// [["\u{0001F469}", "\u{200D}"], ["\u{0001F469}", "\u{200D}"], ["\u{0001F467}", "\u{200D}"], ["\u{0001F466}"]]

let family2 = family1.replacingFirstOccurrence(of: "????", with: "????")
print(family2) // ????‍????‍????‍????
print(family2.characters.map { Array(String($0).unicodeScalars) })
// [["\u{0001F469}", "\u{200D}"], ["\u{0001F469}", "\u{200D}"], ["\u{0001F466}", "\u{200D}"], ["\u{0001F466}"]]

下面是一个可能的版本，它可以定位并替换任意字符串的 Unicode 标量：

extension String {
    func replacingFirstOccurrence(of target: String, with replacement: String) -> String {
        let uc = self.unicodeScalars
        let tuc = target.unicodeScalars

        // Target empty or too long:
        if tuc.count == 0 || tuc.count > uc.count {
            return self
        }

        // Current search position:
        var pos = uc.startIndex
        // Last possible position of `tuc` within `uc`:
        let end = uc.index(uc.endIndex, offsetBy: tuc.count - 1)

        // Locate first Unicode scalar
        while let from = uc[pos..<end].index(of: tuc.first!) {
            // Compare all Unicode scalars:
            let to = uc.index(from, offsetBy: tuc.count)
            if !zip(uc[from..<to], tuc).contains(where: { $0 != $1 }) {
                let prefix = uc[uc.startIndex..<from]
                let suffix = uc[to ..< uc.endIndex]
                return "\(prefix)\(replacement)\(suffix)"
            }
            // Next search position:
            uc.formIndex(after: &pos)
        }

        // Target not found.
        return self
    }
}