Try
String regex = "/foo/(?!.*bar).+";
or possibly
String regex = "/foo/(?!.*\\bbar\\b).+";
to avoid failures on paths like /foo/baz/crowbars which I assume you do want that regex to match.
Explanation: (without the double backslashes required by Java strings)
/foo/ # Match "/foo/"
(?! # Assert that it's impossible to match the following regex here:
.* # any number of characters
\b # followed by a word boundary
bar # followed by "bar"
\b # followed by a word boundary.
) # End of lookahead assertion
.+ # Match one or more characters
\b, the “word boundary anchor”, matches the empty space between an alphanumeric character and a non-alphanumeric character (or between the start/end of the string and an alnum character). Therefore, it matches before the b or after the r in "bar", but it fails to match between w and b in "crowbar".
Protip: Take a look at http://www.regular-expressions.info – a great regex tutorial.