Deprecated: Array and string offset access syntax with curly braces is deprecated

Question

    function _doHeaders_callback_setext($matches) {
    # Terrible hack to check we haven't found an empty list item.
    if ($matches[2] == '-' && preg_match('{^-(?: |$)}', $matches[1]))
        return $matches[0];

    $level = $matches[2]{0} == '=' ? 1 : 2; //here is the 'error'       

    $block = "<h$level>".$this->runSpanGamut($matches[1])."</h$level>";
    return "\n" . $this->hashBlock($block) . "\n\n";
}

Is it possible to disable this message? How could I fix it?