Today I used the regular expression

<?php

    // https://twitter.com/marsroverdriver/status/346776078645342208
    for ($test = 1 << 16; 0 !== $test; --$test)
    {
        $text = '';
        for ($length = rand(8, 10); 0 !== $length; --$length)
        {
            if (0 === rand(0, 1))
            {
                $text .= 'o';
            }
            else
            {
                $text .= chr(rand(ord('a'), ord('z')));
            }
        }
        $reference = preg_match('/^[^o][^o]o[^o][^o]o[^o]o[^o][^o]$/', $text);
        $result = preg_match('/^[^o](?:[^o]o[^o]){2}o[^o]{2}$/', $text);
        if ($reference !== $result)
        {
            echo 'Test failed', PHP_EOL, $text, PHP_EOL;
            var_dump($result);
            var_dump($reference);
            die();
        }
    }
    echo 'Test passed', PHP_EOL;

?>