escape the brackets using a backslash \
http://php.net/manual/en/regexp.reference.meta.php

$conf['sync_chars_regex'] = '/^[[:alpha:][:digit:]-_.\sÀÁÂÃÄÅàáâãäåÒÓÔÕÖØòóôõöøÈÉÊËèéêëÇçÌÍÎÏìíîïÙÚÛÜùúûüÿÑñ!\(\)\&\/\°\'\,\[\]]+$/'; might work