Replace HTML

func replaceHTML(_ str: String, _ pattern: String) ->String{
    guard pattern.count > 0 && str.count > 0 else {
        return str
    }
    var res = ""
    let arr = Array(str)
    let tar = Array(pattern)
    var runner = 0
    while runner < str.count{
        if arr[runner] == "<"{
            let left = runner
            while runner < str.count && arr[runner] != ">"{
                runner += 1
            }
            res.append(String(arr[left ... runner]))
        }else{
            if arr[runner] == tar[0]{
                var left = runner
                var i = 0
                while left < arr.count && i < tar.count && arr[left] == tar[i]{
                    left += 1
                    i += 1
                }
                if i == tar.count{ //means all match
                    res.append("<b>\(pattern)</b>")
                    runner = left
                    continue
                }
            }
            res.append(arr[runner])
        }
        runner += 1
    }

    return res
}

let str = "<span> hello </sellpan> <span>How are you</span>"
print(str)
print(replaceHTML(str, "ell"))