If indeed you are looking for .jpg files under http://b.thumbs.redditmedia.com/ that do not have png in their names, it might be easiest to just get rid of your lookaheads and lookbehinds

my @urls=$response_body =~ m{(http://b.thumbs.redditmedia.com/.+?\.jpg)}gi;

and then add another line to get rid of the lines in the just created @urls that have png in them. Writing regexes that must match one thing but cannot match another are prone to caveats or are not blazingly intuitive.

In reply to Re^2: regex problem by ExReg
in thread regex problem by grasshopper!!!

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