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.

In reply to Re: regex issue (positive look-ahead) by rubasov
in thread regex issue (positive look-ahead) by jeanluca

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