A simple extension to unescape strings that were erroneusly double-escaped for JSON

unescape-json.swift

extension String {
    private static let excapesRegex: Regex = /\\(?<a>[bfnrtv0])|\\(?<x>x\H{2})|(?<u>(?:\\u\H{4})+)/

    struct InvalidEscapeSequenceError: Error {
        let value: Substring
    }
    
    func unescaped() throws -> String {
        try self.replacing(Self.excapesRegex) { match in
            if let a = match.a {
                switch a {
                case "b":
                    "\u{8}"
                case "f":
                    "\u{C}"
                case "n":
                    "\n"
                case "r":
                    "\r"
                case "t":
                    "\t"
                case "v":
                    "\u{B}"
                case "0":
                    "\0"
                default:
                    String(a)
                }
            } else if let x = match.x {
                String(UInt8(x, radix: 16)!) // regex guarantees valid hex
            } else if let u = match.u {
                u.split(separator: "\\u")
                    .map { unichar($0, radix: 16)! /*regex guarantees valid hex*/ }
                    .withUnsafeBufferPointer { String(utf16CodeUnits: $0.baseAddress!, count: $0.count) }
            } else {
                throw InvalidEscapeSequenceError(value: match.0)
            }
        }
    }
}