customStringFormat.swift

import Foundation
import SwiftUI

struct CustomStringFormat {

    let formatString: String
    let fragments: [FragmentType]

    init(formatString: String) {
        self.formatString = formatString
        self.fragments = Self.fragmentizeFormattedString(formatString)
    }

    static func fragmentizeFormattedString(_ formatString: String) -> [FragmentType] {
        return getFragments(regex: "%\\d+\\$@", inputText: formatString)
    }

    static func getFragments(regex: String, inputText: String) -> [FragmentType] {

        var fragments: [FragmentType] = []

        guard let regex = try? NSRegularExpression(pattern: regex) else {
            return []
        }
        let results = regex.matches(in: inputText,
                                range: NSRange(inputText.startIndex..., in: inputText))

        var previousIndex = inputText.startIndex

        var currentIndex = 0
        for match in results {
            for range in 0..<match.numberOfRanges {
                let rangeBounds = match.range(at: range)

                guard let range = Range(rangeBounds, in: inputText) else {
                    continue
                }

                if range.lowerBound != inputText.startIndex {
                    let closedRange = previousIndex..<range.lowerBound
                    fragments.append(.literal(String(inputText[closedRange])))
                }
                fragments.append(.argument(currentIndex))
                previousIndex = range.upperBound
                currentIndex += 1
            }
        }

        if previousIndex < inputText.endIndex {
            fragments.append(.literal(String(inputText[previousIndex..<inputText.endIndex])))
        }

        return fragments
    }
}

enum FragmentType {
    case literal(String)
    case argument(Int)
}

class Example {
    init() {
        let (formattedString, arguments) = Self.getFormattedStringAndArguments()

        let customStringFormat = CustomStringFormat(formatString: formattedString)

        let output = customStringFormat.fragments.map { fragment in
            switch fragment {
            case .literal(let staticText):
                return Text(staticText).foregroundColor(Color.red).font(.largeTitle)
            case .argument(let index):
                return Text(arguments[index]).foregroundColor(Color.yellow).font(.title)
            }
        }.reduce(Text(verbatim: ""), +)

        print(output)
    }

    static func getFormattedStringAndArguments() -> (String, [String]) {
        return ("Going from %1$@ to %2$@. Have a happy journey passing via %3$@. Bon Voyag", ["Boston", "New York", "Providence"])
    }
}

Example,

struct Example: View {

    var body: some View {
        VStack {
            getFormattedTextView(formattedString: "Going from %1$@ to %2$@. Have a happy journey passing via %3$@. Bon Voyag", arguments: ["Boston", "New York", "Providence"])

            getFormattedTextView(formattedString: "%1$@ to %2$@. Have a happy journey passing via %3$@]", arguments: ["Boston", "New York", "Providence"])

            getFormattedTextView(formattedString: "hello %1$@ to %2$@. Have a happy journey passing via %3$@. Bon Voyag", arguments: ["Boston", "New York", "Providence"])
        }
    }

    @ViewBuilder
    func getFormattedTextView(formattedString: String, arguments: [String]) -> some View {

        let customStringFormat = CustomStringFormat(formatString: formattedString)

        let output = customStringFormat.fragments.map { fragment in
            switch fragment {
            case .literal(let staticText):
                return Text(staticText).foregroundColor(Color.red).font(.largeTitle)
            case .argument(let index):
                return Text(arguments[index]).foregroundColor(Color.yellow).font(.title)
            }
        }.reduce(Text(verbatim: ""), +)

        output
    }
}