Swift String extension inserting separator between symbol groups

String+insert.swift

import Foundation

extension String {
    mutating func insert(separator: String, every n: Int) {
        self = inserting(separator: separator, every: n)
    }
    
    func inserting(separator: String, every n: Int) -> String {
        var result: String = ""
        let characters = Array(self)
        stride(from: 0, to: count, by: n).forEach {
            result += String(characters[$0..<min($0+n, count)])
            if $0+n < count {
                result += separator
            }
        }
        return result
    }
}