If you enter something like this (test in german wikipedia):
"5 AND Dezember" you get:
1064: You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND (MATCH (si_title) AGAINST ('dezember')) ) AND cur_namespace":
SELECT cur_id,cur_namespace,cur_title,cur_text FROM cur,searchindex WHERE cur_id=si_page AND ( AND (MATCH (si_title) AGAINST ('dezember')) ) AND cur_namespace IN (0) LIMIT 0, 20
I see two AND's one after the other which means that "this->mTextcond" is emtpy (in the source code). It works with every single character as search term, not only numbers.
Someone good at SearchEngine.php should take a look.