protected function callback($row)
    {
        foreach ($this->getConfig('callbacks') as $column => $callbacks) {
            if (strpos($column, '/') === 0) {
                // Interpret as a regex and apply to all matching columns.
                foreach (array_keys($row) as $name) {
                    if (preg_match($column, $name) === 1) {
                        foreach ((array) $callbacks as $callback) {
                            $row[$name] = call_user_func($callback, $row[$name]);
                        }
                    }
                }
            } else {
                // Apply to one explicitly named column.
                if (array_key_exists($column, $row) === false) {
                    throw new \RuntimeException('Unable to apply callback to missing column: ' . $column);
                }
                foreach ((array) $callbacks as $callback) {
                    $row[$column] = call_user_func($callback, $row[$column]);
                }
            }
        }
        return $row;
    }