JS Whitespace stripper

$output = preg_replace_callback(
    '#//[^\n]*|/\*.*?\*/|/(?!\s)(?:\\\\.|[^/\\\\])*/|\'(?:\\\\.|[^\'\\\\])*\'|"(?:\\\\.|[^"\\\\])*"|\s*[^0-9a-z_$\'"/\s]\s*|\s+#si',
    function($m) {
        $m = trim($m[0]);
        return $m === '' ? ' ' : ($m[0] === '/' && ($m[1] === '*' || $m[1] === '/') ? '' : $m);
    },
    $input);