Matteo & Vladimir,
This is the change in liferay 6.0, If you check
friendly url implementation in liferay 6.0, you will get the clue.
As per the journal article route file article id should be the digit only,
please check "journal-content-friendly-url-routes.xml", for more details.
<route>
<pattern>/56/{groupId:\d+}/
{articleId:\d+}</pattern>
<implicit-parameter name="instanceId">0000</implicit-parameter>
<implicit-parameter name="p_p_state">maximized</implicit-parameter>
<implicit-parameter name="struts_action">/journal_content/view</implicit-parameter>
</route>
"\d+" is regular expression to specify the format of parameters,
Hope this may help you.
Let me know, if you wants more information on same.
Thanks & Regards,
Samir Gami