Need help creating a irule to reverse proxy .

If I read this correctly, when the customer sends http://www.lightsource.com/blog, you want the backend server to see what is essentially http://lightsource.wpengine.com. The be pedantic, you want the Host: header to change from www.lightsource.com to lightsource.wpengine.com and the request-uri to change from /blog to /. You do not want to perform an HTTP redirect because you want this change to be transparent to the user-agent.

You can solve this on 11.4 or higher using a Local Traffic Policy:

• SOL15085: Overview of the Local Traffic Policies feature

Here is an example:

ltm policy lightsource-rewrite {
    controls { forwarding }
    requires { http }
    rules {
        rule01 {
            actions {
                0 {
                    http-host
                    replace
                    value lightsource.wpengine.com
                }
                1 {
                    http-uri
                    replace
                    path /
                }
            }
            conditions {
                0 {
                    http-host
                    host
                    values { www.lightsource.com }
                }
                1 {
                    http-uri
                    path
                    values { /blog }
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}

Alternatively, you can do the same in an iRule:

when HTTP_REQUEST {
    if { [string tolower [HTTP::host]] eq "www.lightsource.com" and [HTTP::path] eq "/blog" } {
        HTTP::replace Host lightsource.wpengine.com
        HTTP::path "/"
    }
}