Codeigniter custom helper for bbcode, apply function on parameter

I made a custom bbcode parser function and added it on my helper

if ( !function_exists('bbcode_parser'))
{
    function bbcode_parser($str) 
    {
        $patterns = array(
            '#\[b\](.*?)\[/b\]#is',
            '#\[img\](.*?)\[/img\]#is',
            '#\[url\](.*?)\[/url\]#is',
            '#\[url=(.*?)\](.*?)\[/url\]#is'
        );

        $replacements = array(
            '<strong>$1</strong>',
            '<img src="$1" />',
            '<a href="$1">$1</a>',
            '<a href="$1">$2</a>',
        );

        $str = preg_replace($patterns, $replacements, $str);
        return $str;
    }
}

It's good and works like I want it to but my question is how to apply a function on each replacement value.

fe. for the url that doesn't have inner data, i want to replace with the website title of the url

or validate the url if it has http://

i would also like to check the size of the image, if it's too large, i want to resize it when printed by adding a "width" attribute and then just add an tag to the full size image.

is this possible? if so, how to do this?