help with regex - how can i make some urls no-follow?

Ok so i've made this function which works fine for converting most urls like pies.com or www.cakes.com to an actual link tag.

function render_hyperlinks($str){       
    $regex = '/(http:\/\/)?(www\.)?([a-zA-Z0-9\-_\.]+\.(com|co\.uk|org(\.uk)?|tv|biz|me)(\/[a-zA-Z0-9\-\._\?&=#\+;]+)*)/ie';    
    $str = preg_replace($regex,"'<a href=\"http://www.'.'$3'.'\" target=\"_blank\">'.strtolower('$3').'</a>'", $str);
    return $str;    
}

I would like to update this function to add no-follow tags to links to my competitors,

so i would have certain keywords (competitor names) to nofollow for example if my site was about baking i might want to:

no-follow any sites with the phrases 'bakingbrothers', 'mr开发者_高级运维kipling', 'lyonscakes'

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is it possible to implement this if(contains x){ add y} into my regex?

is this what is called a 'lookback'?

---

Maybe preg_replace_callback is what you are looking for:

function link($matches)
{
    $str_return = '<a href="http://www.'.$matches[3].'" target="_blank"';
    if(in_array($matches[3], $no_follow_array))
    {
        $str_return .= ' no-follow';
    }
    $str_return .='>'.strtolower($matches[3]).'</a>';
}

$regex = '/(http:\/\/)?(www\.)?([a-zA-Z0-9\-_\.]+\.(com|co\.uk|org(\.uk)?|tv|biz|me)(\/[a-zA-Z0-9\-\._\?&=#\+;]+)*)/ie';    
$str = preg_replace_callback($regex,'link', $str);