embeddedString = embeddedString.replace(/(widths*=s*["'])[0-9]+(["'])/ig, $1 + yourWidth + $2);
embeddedString = embeddedString.replace(/(heights*=s*["'])[0-9]+(["'])/ig, $1 + yourHeight + $2);

embeddedString = embeddedString.replace(/widths*=s*["']["']/ig, function($0, $1)
{
    return $1 * 2;
});

var originalWidth;
var originalHeight;
embeddedString = embeddedString.replace(/(?:width|height)s*=s*["']([0-9]+)["']s*(?:width|height)s*=s*["']([0-9]+)["']/ig, function($0, $1, $2, $3, $4)
{
    // $0 = original string
    // $1 = either 'width' or 'height'
    // $2 = the value of width or height, depending on above
    // $3 = either 'width' or 'height'
    // $4 = the value of width or height, depending on above
    // here you might want to set another value, eg:
    originalWidth = $2;
    return "";
});

width="(.*?)" height="(.*?)"

$pattern = '/width="(.*?)" height="(.*?)"/';
     $subject = 'width="500" height="300"';

     preg_match($pattern, $subject, $matches);

     print_r($matches);