Regular expression to match string not containing a word?

Question

I know it is possible to match for the word and using tools options reverse the match. (eg. by grep -v) However I want to know if it is possible using regular expressions to match lines which does not contain a specific word, say hede?

Input:

Hoho
Hihi
Haha
hede

# grep "Regex for do not contain hede" Input

Output:

Hoho
Hihi
Haha

Answer 1

If you're just using it for grep, you can use grep -v hede to get all lines which do not contain hede.

ETA Oh, rereading the question, grep -v is probably what you meant by "tools options".

Answer 2

Here's a good explanation of why it's not easy to negate an arbitrary regex. I have to agree with the other answers, though: if this is anything other than a hypothetical question, then a regex is not the right choice here.

Answer 3

The fact that regex doesn't support inverse matching is not entirely true. You can mimic this behavior by using negative look-arounds:

^((?!hede).)*$

The regex above will match any string, or line without a line break, not containing the (sub) string 'hede'. As mentioned, this is not something regex is "good" at (or should do), but still, it is possible.