pretty basic question about an irule (I think)

I need to write an iRule that looks for a specific string and if it appears, remove angle brackets from the payload. So far I have this:

when CLIENT_DATA {
    set haystack[string tolower [TCP::payload]]
    if { $haystack contains "needle" } {

    }
} 

How do I go about replacing the contents of $haystack, and then releasing the payload to the server?