Regular Expression to replace all HTML tags

I wrote a regular expression, <(. | \ n) +?>
This is a replacement of HTML tags, non-greedy, multi-line.

 

If I want to replace it with a non-HTML tag,
This is the only way for my code to do this. First, find the HTML Tag and replace the tag.

 

Can I find a non-HTML Tag directly ..

 

Another problem is that the length of the string is intercepted.
The following method does not judge Chinese or non-Chinese characters. The length of the screenshot is always long or short.
I don't know if there is any better way to get the Length of the clip, which is the same Length, rather than the Length of str. Length.

    public static string formatString(string str, int size)
    {
string temp = str;
//Encoding.Default.GetBytes(str).Length;
        Regex regex = new Regex("<.+?>");
        temp = regex.Replace(str, "");
//for (int i = 0; i < regex.Matches(str).Count; i++)
//{
//    temp = temp.Replace(regex.Matches(str)[i].Value, "");
//}
        temp = temp.Replace("\r\n", "");
        temp = temp.Replace("&nbsp;", "");
if (temp.Length >= size)
        {
            temp = temp.Substring(0, size - 3) + "";
        }
return temp;
    }