findstr - sub string of last occurrence of a character to the end of the string

set str "XXX-GHDF-12XD45"
set sub_str [string range $str [string last "-" $str]+1 end]

or

set str "XXX-GHDF-12XD45"
set sub_str [lindex [set str_list [split $str "-"]] [llength $str_list]-1]

You can use

getfield

Example: XXX-GHDF-12XD45

        set eidval {XXX-GHDF-12XD45}
        set val [getfield $eidval "-" 3]  

https://devcentral.f5.com/wiki/irules.findstr.ashx

OR

        set spr "XXXA-GHDF-12XD45"
        set sub_spr [string range $spr [string last "-" $spr]+1 end

set listOfStr [split "XXX-GHDF-12XD45" "-"]
set location [expr {[llength $listOfStr] - 2}
set output [lindex $listOfStr $location]