sapply = "!f() { git stash apply \"$(git stash list | awk -F: --posix -vpat=\"$*\" \"$ 0 ~ pat {print $ 1; exit}\")\"; }; f"
git sapply "<regex>"
Edit: I sticked to my original solution, but I see why majority would prefer Etan Reisner's version (above). So just for the record:
sapply = "!f() { git stash apply \"$(git stash list | grep -E \"$*\" | awk \"{ print $ 1; }\" | sed -n \"s/://;1p\")\"; }; f"