Minifying final HTML output using regular expressions with CodeIgniter

Question

Google pages suggest you to minify HTML, that is, remove all the unnecessary spaces. CodeIgniter does have the feature of giziping output or it can be done via .htaccess. But still I also would like to remove unnecessary spaces from the final HTML output as well.

I played a bit with this piece of code to do it, and it seems to work. This does indeed result in HTML that is without excess spaces and removes other tab formatting.

class Welcome extends CI_Controller 
{
    function _output()
    {
        echo preg_replace('!\s+!', ' ', $output);
    }

    function index(){
    ...
    }
}

The problem is there may be tags like <pre>,<textarea>, etc.. which may have spaces in them and a regular expression should remove them. So, how do I remove excess space from the final HTML, without effecting spaces or formatting for these certain tags using a regular expression?

Thanks to @Alan Moore got the answer, this worked for me

echo preg_replace('#(?ix)(?>[^\S ]\s*|\s{2,})(?=(?:(?:[^<]++|<(?!/?(?:textarea|pre)\b))*+)(?:<(?>textarea|pre)\b|\z))#', ' ', $output);

ridgerunner did a very good job of analyzing this regular expression. I ended up using his solution. Cheers to ridgerunner.