sed: Can my pattern contain an “is not” character? How do I say “is not X”?

Question

How do I say \"is not\" a certain character in sed?

Answer 1

For those not satisfied with the selected answer as per johnny's comment.

'su[^x]' will match 'sum' and 'sun' but not 'su'.

You can tell sed to not match lines with x using the syntax below:

sed '/x/! s/su//' file

See kkeller's answer for another example.