Irule advice?

when HTTP_REQUEST {
    if { [string tolower [HTTP::query]] contains "fld" } {
        if { ![string is digit [URI::query [HTTP::uri] "fld"]] } {
            log local0. "invalid fld value, rejecting from [IP::client_addr]"
            reject
        }
    }
}

The following accounts for a POST request where the payload is URL encoded or XML:

when HTTP_REQUEST { 
    if { [HTTP::method] eq "POST" } { 
        ## Trigger collection for up to 1MB of data 
        if { [HTTP::header exists "Content-Length"] && [HTTP::header "Content-Length"] <= 1048576 }{ 
            set content_length [HTTP::header "Content-Length"] 
        } else { 
            set content_length 1048576 
        } 
        ## Check if $content_length is not set to 0 
        if { $content_length > 0 } { 
            HTTP::collect $content_length 
        } 
    } 
} 
when HTTP_REQUEST_DATA { 
    set fld ""
    if { [HTTP::payload] contains "fld=" } {
        foreach x [split [HTTP::payload] "&"] {
            if { $x starts_with "fld=" } {
                set fld [lindex [split $x "="] 1]
                continue
            }
        }
    } elseif { [HTTP::payload] contains "<fld>" } {
        set fld [findstr [HTTP::payload] "<fld>" 5 "</fld>"]
    }
    if { $fld ne "" } {
        if { ![string is digit $fld] } {
            log local0. "invalid fld value, rejecting from [IP::client_addr]"
            HTTP::respond 400 content "Bad Request" "Content-Type" "text/html" "Connection" "close"
        }
    }
}