ROPE Language Reference - "expect"

The "expect" action takes a specified number of bytes from the packet being inspected, starting at $offset. The lifted bytes are returned as a string and $offset is advanced to point to the next byte in the packet.

If there are fewer than the requested number of bytes available, expect returns a "no" status to NetFilter, indicating that the packet being inspected does not match.

The "lift" action is similar, but handles the end-of-packet differently

For example..

expect(4)
4 expect

See Also