protected function _findEndOfURL($start = 0) {
        $length = strlen($this->_css);
        $index = $start + 4;

        while ($index < $length) {
            $next = substr($this->_css, $index, 1);
            switch ($next) {
                case '"':
                case '\'':
                    // quoted url
                    $end = $this->_findEndOfQuotedString($next, $index);
                    if ($end === false) {
                        return new ParseError('Incomplete string', $this->_css, $index);
                    }
                    $end = strpos($this->_css, ')', $end);
                    return $end === false ? new ParseError('Cannot find end of URL', $this->_css, $start) : $end + 1;

                case ' ':
                case "\t":
                case "\r":
                case "\n":
                    // skip whitespace
                    $index ++;
                    break;

                default:
                    // unquoted url
                    while (true) {
                        switch ($next) {
                            case ')':
                                return $index + 1;

                            case '"':
                            case '\'':
                            case '(':
                            case ' ':
                            case "\t":
                            case "\r":
                            case "\n":
                                return new ParseError('Invalid URL', $this->_css, $start);

                            default:
                                if (ord(substr($this->_css, $index, 1)) < 32) {
                                    return new ParseError('Invalid URL', $this->_css, $start);
                                }
                        }
                        $index ++;
                        if ($index >= $length) {
                            return new ParseError('Cannot find end of URL', $this->_css, $start);
                        }
                        $next = substr($this->_css, $index, 1);
                    }
            }
        }
        return new ParseError('Cannot find end of URL', $this->_css, $start);
    }