The glob pattern did not work (probably issues with the negation).
RE2 (Google's high performance RegEx engine) does not support negation either, that's why it has to be expanded this way (negated character groups are supported).
Glob patterns are not matched against the leading slash, but regexes are (undocumented, but observed).
Relates to !43 and therefore ae3148337b (the behavior finaly matches the description).
My reasoning is:
* any CSS or JS files
their name contains their hash — 1 month
* anything under img folder
might change while keeping name — 1 day
* any image anywhere except img folder
usually giant photos attached to blog posts, rarely change and if they do, they usually change name as well — 1 month
