Refactoring in Swift: Extract Method for Improved Code Maintainability

func processUserRegistration(userData: [String: Any]) -> [String: Any] {
    do {
        guard let username = userData["username"] as? String, username.count >= 3, username.count <= 20 else {
            return [
                "success": false,
                "message": "Invalid username. Must be between 3 and 20 characters."
            ]
        }

        guard let email = userData["email"] as? String, email.contains("@"), email.contains(".") else {
            return ["success": false, "message": "Invalid email format."]
        }

        guard let password = userData["password"] as? String, password.count >= 8,
              password.rangeOfCharacter(from: .uppercaseLetters) != nil,
              password.rangeOfCharacter(from: .decimalDigits) != nil,
              password.rangeOfCharacter(from: CharacterSet(charactersIn: "!@#$%^&*")) != nil else {
            return [
                "success": false,
                "message": "Password must be at least 8 characters and contain uppercase, number, and special character."
            ]
        }

        guard let dateOfBirthString = userData["date_of_birth"] as? String,
              let birthDate = ISO8601DateFormatter().date(from: dateOfBirthString) else {
            return [
                "success": false,
                "message": "Invalid date of birth format."
            ]
        }
        
        let today = Date()
        let ageComponents = Calendar.current.dateComponents([.year], from: birthDate, to: today)
        guard let age = ageComponents.year, age >= 18, age <= 120 else {
            return [
                "success": false,
                "message": "Invalid date of birth or user must be 18+"
            ]
        }

        guard let address = userData["address"] as? [String: String],
              let street = address["street"], street.count >= 5,
              let city = address["city"], city.count >= 2,
              let country = address["country"], country.count >= 2,
              let postalCode = address["postal_code"], postalCode.count >= 3, postalCode.rangeOfCharacter(from: .alphanumerics) != nil else {
            return ["success": false, "message": "Invalid address details."]
        }

        let userId = "USER_" + (0..<9).map { _ in "abcdefghijklmnopqrstuvwxyz0123456789".randomElement()! }.joined()

        let normalizedData: [String: Any] = [
            "username": username.lowercased(),
            "email": email.lowercased(),
            "password": password,
            "date_of_birth": ISO8601DateFormatter().string(from: birthDate),
            "address": [
                "street": street.trimmingCharacters(in: .whitespaces),
                "city": city.trimmingCharacters(in: .whitespaces),
                "country": country.uppercased(),
                "postal_code": postalCode.uppercased()
            ]
        ]

        // Assuming dataStore is a property of the class
        try dataStore.store(normalizedData)

        return [
            "success": true,
            "message": "User registered successfully",
            "user_id": userId
        ]
    } catch {
        return [
            "success": false,
            "message": "Registration failed: \(error.localizedDescription)"
        ]
    }
}