Answer:
^((?!hede).)*$
Explanation:
^
the beginning of the string,
(
group and capture to \1 (0 or more times (matching the most amount possible)),
(?!
look ahead to see if there is not,
hede
your string,
)
end of look-ahead,
.
any character except \n,
)*
end of \1 (Note: because you are using a quantifier on this capture, only the LAST repetition of the captured pattern will be stored in \1)
$
before an optional \n, and the end of the string