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Parsing markup with regex is like building your house using lego... it's not the right tool for the job. HTML is not a regular language, therefore regular expressions don't cut the mustard.
What you need is a DOM parser, and as luck would have it, PHP has the DOMDocument object, which is just that:

$dom = new DOMDocument;
$dom->loadHTML('<img onload="alert(\'hello world\');" onclick="alert(\'hello world\');" />');
$nodes = $dom->getElementsByTagName('*');//just get all nodes, 
//$dom->getElementsByTagName('img'); would work, too
foreach($nodes as $node)
{
    if ($node->hasAttribute('onload'))
    {
        $node->removeAttribute('onload');
    }
    if ($node->hasAttribute('onclick'))
    {
        $node->removeAttribute('onclick');
    }
}
echo $dom->saveHTML();//will include html, head, body tags and doctype

Tadaa... both onload and onclick have been removed from the markup, without the pain of writing a reliable and stable regex, that can deal with in-line JS... As an added bonus, this code will be far more maintainable (and expandable) in the future. I'd much prefer maintaining this code, than having to rework a regular expression somebody wrote a couple of months ago...

If you want, you can echo only the tags you've changed, like so:

$changed = array();
$attributesOfDeath = array('onload', 'onclick');
foreach($nodes as $node)
{
    $current = null;
    foreach($attributesOfDeath as $attr)
    {
        if ($node->hasAttribute($attr))
        {
            $node->removeAttribute($attr);
            $current = $node;
        }
    }
    if ($current)
    {
        $changed[] = $current;//add to changed array
    }
}
$changed = array_map(array($dom, 'saveXML'), $changed);
echo implode(PHP_EOL, $changed);