diff --git a/tcshrc.prompt.defs b/tcshrc.prompt.defs
index d552766..e76b7df 100644
--- a/tcshrc.prompt.defs
+++ b/tcshrc.prompt.defs
@@ -140,7 +140,9 @@ _debug "Building Cool prompt"
 set depth_prompt="(${shlvl})"
 
 #we "convert" pts/$number entries to ttypK entries?
-set line_prompt="`tty | sed -e 's;/dev/tty;;' -e 's;/dev/pts/;p/;' -e 's;/dev/;;' -e 's/not a tty/${unknown_tty}/'`"
+set line_prompt=`tty`
+set line_prompt=${line_prompt:s;/dev/tty/;;:s;/dev/pts/;p/;:s;/dev/;;:s/not a tty/?/}
+
 #set line_prompt="line"
 set user_prompt="%n"
 set host_prompt="${CSHENV_HOSTNAME}"