Replace substring using regex in a string

What was not working ?

Problem is that once you replace the first match, the second position's is wrong. Just start from the end.

I wrote far2cel and cel2far, and that works:

let tempMsg = "This is today. The temperature is around 14C. Tomorrow the temperature will be around 18C"

func far2cel(_ val: Double) -> String {
    return String((val - 32) / 1.8) + "C"
}

func cel2far(_ val: Double) -> String {
    return String((val * 1.8) + 32) + "F"
}

func formatCorrectDetail(_ detail: String, toCelsius: Bool) -> String { // changed for toCelsius, more readable
    let regex = try! NSRegularExpression(pattern: "around ((\\d+)" + (toCelsius ? "F" : "C") + ")", options: [])
    let matches = regex.matches(in: detail, options: [], range: NSRange(location: 0, length: detail.utf16.count))
    let formattedString = NSMutableString(string: detail)

    let reversedMatches = matches.reversed()
    for match in reversedMatches { // <<-- You need to start at the end
      let range = match.range(at: 2)
      if let rangeSubstring = Range(range, in: detail),
        let temperature = Double(String(detail[rangeSubstring])) {
          let rangeForUnit = NSRange(location: range.upperBound, length: 1)  // First remove old unit
          formattedString.replaceCharacters(in: rangeForUnit, with: "")

          let val = toCelsius ? far2cel(temperature) : cel2far(temperature)
        formattedString.replaceCharacters(in: range, with: val)
      }
    }

    if formattedString.length > 0 {
      return String(formattedString)
    }
    else {
      return detail
    }
  }

let newTempMsg = formatCorrectDetail(tempMsg, toCelsius: false)
print(newTempMsg)

That gives: This is today. The temperature is around 57.2F. Tomorrow the temperature will be around 64.4F

Looks like my implementation seems off

static func roundOff(_ val: String, _ decimalPlace: Int) -> String {
    let decimalFormatter = createDecimalFormatter()
    decimalFormatter.minimumFractionDigits = decimalPlace
    decimalFormatter.maximumFractionDigits = decimalPlace

    let bigDecimal = NSDecimalNumber(string: val)
    return decimalFormatter.string(from: bigDecimal.rounding(accordingToBehavior: NSDecimalNumberHandler(roundingMode: .up, scale: Int16(decimalPlace), raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: false)))!
  }

static func cel2far(_ celsius: Double) -> String {
    return roundOff(String(celsius * 9.0 / 5.0 + 32.0), 0)
  }

  static func far2cel(_ fahrenheit: Double) -> String {
    return roundOff(String((fahrenheit - 32.0) * 0.5555555555555556), 0)
  }

LOL

Is it solved now ? If not, could you explain what the problem is precisely ? If solved, don't forget to close the thread.

Note: I don't know if problem was in func, but there was an issue with replacements.