Swift's String(format:arguments:) method (peeking under the hood)

String+Formatting.swift

extension String {

   /// Replaces format specifiers (with index) with given arguments.
   /// (Only works if given arguments are Strings and to replace Strings)
   ///
   /// - Parameter arguments: List of String replacements.
   /// - Returns: This String replaced with given Strings (unless arguments are not a String array)
   func replaceFormatSpecifiersWithIndex(with arguments: CVarArg...) -> String {
       guard let args = arguments as? [String] else { return self }

       // ensure patterns are valid
       let formatSpecifierPattern = "%[\\d]+\\$@"
       let digitPattern = "[\\d]+"
       guard
           let formatSpecifierRegEx = try? NSRegularExpression(pattern: formatSpecifierPattern, options: .useUnixLineSeparators),
           let digitRegEx = try? NSRegularExpression(pattern: digitPattern, options: .useUnixLineSeparators)
       else { return self }

       // extract range of format specifiers (if any)
       var range = NSRange(location: 0, length: self.count)
       var matches = formatSpecifierRegEx.matches(in: self, options: .withoutAnchoringBounds, range: range)
       let mutableSelf = NSMutableString(string: self)

       while matches.count > 0 {

           // extract index from format specifier
           guard let matchingFormatSpecifierRange = matches.first?.range else { break }
           let formatSpecifier = mutableSelf.substring(with: matchingFormatSpecifierRange)
           let digitRange = NSRange(location: 0, length: formatSpecifier.count)
           let digitMatches = digitRegEx.matches(in: formatSpecifier, options: .withoutAnchoringBounds, range: digitRange)

           guard let matchingDigitRange = digitMatches.first?.range else { break }
           let mutableFormatSpecifier = NSMutableString(string: formatSpecifier)
           let index = NSDecimalNumber(string: mutableFormatSpecifier.substring(with: matchingDigitRange)).intValue - 1

           // try to replace string
           let replacementString = index < args.count ? args[index] : "(null)"
           mutableSelf.replaceCharacters(in: matchingFormatSpecifierRange, with: replacementString)

           // overwrite range & matches to reflect newly replaced string
           range = NSRange(location: 0, length: mutableSelf.description.count)
           matches = formatSpecifierRegEx.matches(in: mutableSelf.description, options: .withoutAnchoringBounds, range: range)
       }

       return mutableSelf.description
   }

   /// Replaces format specifiers (without index) with given arguments.
   /// (Only works if given arguments are Strings and to replace Strings)
   ///
   /// - Parameter arguments: List of String replacements.
   /// - Returns: This String replaced with given Strings (unless arguments are not a String array)
   func replaceFormatSpecifierWithoutIndex(with arguments: CVarArg...) -> String {
       guard let args = arguments as? [String] else { return self }

       // ensure pattern is valid
       let formatSpecifierPattern = "%@"
       guard let regEx = try? NSRegularExpression(pattern: formatSpecifierPattern, options: .useUnixLineSeparators) else { return self }

       // extract range of format specifiers (if any)
       var range = NSRange(location: 0, length: self.count)
       var matches = regEx.matches(in: self, options: .withoutAnchoringBounds, range: range)
       let mutableSelf = NSMutableString(string: self)
       var index = 0

       while matches.count > 0 {

           // try to replace string
           guard let matchingFormatSpecifierRange = matches.first?.range else { break }
           let replacementString = index < args.count ? args[index] : "(null)"
           mutableSelf.replaceCharacters(in: matchingFormatSpecifierRange, with: replacementString)

           // overwrite range & matches to reflect newly replaced string
           range = NSRange(location: 0, length: mutableSelf.description.count)
           matches = regEx.matches(in: mutableSelf.description, options: .withoutAnchoringBounds, range: range)

           index += 1
       }

       return mutableSelf.description
   }
}