Help with Scan or String first last and multiple/last occurence

I don't think you necessarily can use scan, because you won't know how many dashed values there are before the period. This is what I might do:

set test [list]
lappend test "prod-tools-discovery.acme.com"
lappend test "prod-tools.acme.com"
lappend test "prod-io-tools.acme.com"
lappend test "stg-io-tools-config.acme.com"
lappend test "stg-io-tools.acme.com"

foreach x $test {
    log local0. "input = [lindex [split $x "."] 0]"
    set a [lindex [split $x "."] 0]
    set b [lindex [split $a "-"] [expr [llength [split $a "-"]] -1]]
    log local0. $b
}

I suppose you technically could use scan, but I think it'd be more troublesome:

set test [list]
lappend test "prod-tools-discovery.acme.com"
lappend test "prod-tools.acme.com"
lappend test "prod-io-tools.acme.com"
lappend test "stg-io-tools-config.acme.com"
lappend test "stg-io-tools.acme.com"

foreach x $test {
    catch { unset a b c d }
    scan [lindex [split $x "."] 0] {%[^-]-%[^-]-%[^-]-%s} a b c d
    if { [info exists d] } { 
        log local0. $d
    } elseif { [info exists c] } {
        log local0. $c
    } elseif { [info exists b] } {
        log local0. $b
    } else {
        log local0. $a
    }
}