zsh parameter expansion puzzle : last argument of command

I need to find the last argument of a command, check if it's a valid file name, if so I pass that to fre which is part of my command-completion toolchain. The code I have works except in one case: if the last argument of the command is a valid file name containing embedded spaces. I can't see how to deal with this. Any ideas? Here's the code:

   fre_preexec () {
      local tmpcmd="$1"
      local cand_fname="${tmpcmd##* }"
      [ -f "$cand_fname" ] && fre --add "$cand_fname"
   }

According to the documentation, for preexec $1 contains the entire command string. Thanks in advance.

Edit: I managed to get something that snagged a file name, but I can't get that to past the -f test:

   local tmpcmd="$1"
   local cand_fname="${tmpcmd##*[^\\] }"
   echo "cand_fname ${cand_fname}"
#   [ -f "$cand_fname" ] && fre --add "$cand_fname"
   [ -f "$cand_fname" ] && echo fre --add "$cand_fname"

If the command is ls -l fred\ bill.txt, this prints

cand_fname fred\ bill.txt

but does not echo anything. I can't figure that part out.

You need a mix of the z parameter expansion flag to split a string according to normal shell rules, and the Q flag to handle removing the literal backslash (And other quote related characters) left in the arguments to preexec. For example:

#!/usr/bin/env zsh

preexec () {
    local tmpcmd="$3"
    # Note the nested expansion
    local cand_fname=${(Q)${(z)tmpcmd}[-1]}
    print "cand_fname: $cand_fname"
    [ -f "$cand_fname" ] && echo fre --add "$cand_fname"
}


ls -l fred\ bill.txt

and in action:

$ touch "fred bill.txt"
$ zsh demo.sh
cand_fname: fred bill.txt
fre --add fred bill.txt
-rw-r--r-- 1 x x 0 Feb  7 05:07 'fred bill.txt'