How to check if response contains a specific string using IRules

I may be interpreting your requirement a little differently, but if you're asking to change a (set)cookie value based on some matching content in the response, here's an option:

when HTTP_RESPONSE {
    if { [HTTP::header Content-Type] contains "text" } {
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_RESPONSE_DATA {
    if { [HTTP::payload] contains "diam" } {
        if { [HTTP::cookie exists JSESSIONID] } {
            HTTP::cookie value JSESSIONID "foobar"
        }
    }
}

This can get tricky based on where you might find this data and how large the response payload is. Collecting all of the response payload can cause a performance hit.