NSMutableString Playground - change portions of it based on custom tags

NSMutableStringStrikeThrough.swift

import Foundation
import UIKit

// Say you have a NSMutableString like "<strikeStartTag>+ 6.99<strikeEndTag>" and need to strike through "+ 6.99".

extension String {
    func firstRange(from start: String, to end: String?) -> Range<String.Index>? {
        guard let lower = range(of: start)?.upperBound else { return nil }
        guard let end = end else { return lower..<self.endIndex }
        guard let upper = self[lower...].range(of: end)?.lowerBound else { return nil }
        return lower..<upper
    }
}
extension NSRange {
    init(_ range: Range<String.Index>, in string: String) {
        let location = range.lowerBound.utf16Offset(in: string)
        self.init(
            location: location,
            length: range.upperBound.utf16Offset(in: string) - location
        )
    }
}
extension NSMutableAttributedString {
    func remove(string: String) {
        guard let stringRange = self.string.range(of: string) else { return }
        mutableString.replaceCharacters(in: NSRange(stringRange, in: string), with: "")
    }
}

let strikeStartTag = "<strikeStart>"
let strikeEndTag = "<strikeEnd>"
let additionalCostPrices = NSMutableAttributedString(string: "\(strikeStartTag)+ 6.99\(strikeEndTag)")

if let priceRange = additionalCostPrices.string.firstRange(from: strikeStartTag, to: strikeEndTag) {
    additionalCostPrices.string[priceRange]
    additionalCostPrices.addAttributes(
        [
            NSAttributedString.Key.strikethroughStyle: NSUnderlineStyle.single.rawValue,
            NSAttributedString.Key.strikethroughColor: UIColor.black
        ],
        range: NSRange(priceRange, in: additionalCostPrices.string)
    }
    additionalCostPrices.remove(string: strikeStartTag)
    additionalCostPrices.remove(string: strikeEndTag)
}