how to flush lines that do NOT include characters 'FK'

[Roland Reichenberg]

Hi NG,

in my buffer, I want to flush all lines that do NOT contain the letters "FK"

My buffer contains lots of lines like:
laksjdfh
oqwirutz
095876
mxvb
aelkdjfFKddd
dffff
kkkkss
kkssss
aeskldjfopqirt
iquirmvqkrik
owuiertznvc
q9prskladhf
FK_hhh20894
1s9845FK01
095678jf
ajfhlakjdfh

>From these lines I want to filter the follwing
aelkdjfFKddd
FK_hhh20894
1s9845FK01

Any suggestions for a REGEXP?

I have tried the following:
M-x flush-lines [^FK]

But without success.

Who can help?

Thanks in advance,

        Roland