Конвертируем BBCode в HTML

public string ConvertBBCodeToHTML (string str)
{
    Regex exp;
    // format the bold tags: [b][/b]
    // becomes: 
    exp = new Regex(@"\[b\](.+?)\[/b\]");
    str = exp.Replace(str, "$1");
       
    // format the italic tags: [i][/i]
    // becomes: 
    exp = new Regex(@"\[i\](.+?)\[/i\]");
    str = exp.Replace(str, "$1");
       
    // format the underline tags: [u][/u]
    // becomes: 
    exp = new Regex(@"\[u\](.+?)\[/u\]");
    str = exp.Replace(str, "$1");
              
    // format the strike tags: [s][/s]
    // becomes: 
    exp = new Regex(@"\[s\](.+?)\[/s\]");
    str = exp.Replace(str, "$1");
              
    // format the url tags: [url=www.website.com]my site[/url]
    // becomes: my site
    exp = new Regex(@"\[url\=([^\]]+)\]([^\]]+)\[/url\]");
    str = exp.Replace(str, "$2");
    
    // format the img tags: [img]www.website.com/img/image.jpeg[/img]
    // becomes: 
    exp = new Regex(@"\[img\]([^\]]+)\[/img\]");
    str = exp.Replace(str, "");
    
    // format img tags with alt: [img=www.website.com/img/image.jpeg]this is the alt text[/img]
    // becomes: 
    exp = new Regex(@"\[img\=([^\]]+)\]([^\]]+)\[/img\]");
    str = exp.Replace(str, "");
    
    //format the colour tags: [color=red][/color]
    // becomes: 
    // supports UK English and US English spelling of colour/color
    exp = new Regex(@"\[color\=([^\]]+)\]([^\]]+)\[/color\]");
    str = exp.Replace(str, "$2");
    exp = new Regex(@"\[colour\=([^\]]+)\]([^\]]+)\[/colour\]");
    str = exp.Replace(str, "$2");
    
    // format the size tags: [size=3][/size]
    // becomes: 
    exp = new Regex(@"\[size\=([^\]]+)\]([^\]]+)\[/size\]");
    str = exp.Replace(str, "$2");
    
    // lastly, replace any new line characters with 

    str = str.Replace("\r\n", "
\r\n");
    
    return str;
}