In short: because the lookahead you're using is a zero-width assertion.

In a more explanatory style: your second regex means something like this: match a url part not containing slashes, followed by a slash (but do not include this slash into to the currently matched substring), and followed by the end of the string.

url part not containing slashes -|- followed by a slash
                                 |- followed by the end of the string
[download]

Those followed by clauses should be fulfilled at the same time, however your url does not contain such a substring.

update: corrected a negation error above, thanks ikegami.