Is there anything specific that causes Invalid entries ont he hit_ignore? I was trying to get the must visited articles. I recently noticed that there are a lot of entries with invalid... so I'm wondering if anybody knows what makes a hit "invalid"

for those interested, this is my Query:
SELECT visitURL, referingURL, count( * ) AS count
FROM evo_hitlog
WHERE hit_ignore NOT
IN (
'invalid', 'rss', 'robot'
)
AND visitURL NOT REGEXP '/[0-9]*$'
GROUP BY visitURL
ORDER BY 3 DESC
LIMIT 20

[disclaimer]: I don't know what RegExp'/[0-9]*$' really means. I got this query from one of the plug-ins.

Thanks...