BUG: listpri picked up non-priority patterns.

The regexp for the priority wasn't anchored to the beginning of the task. (As the filtering is done inside the _list pipeline, the task number has already been prepended.)
Also, by passing the regexp directly to _list, a case-insensitive search was performed, so despite [A-Z], lowercase characters were picked up, too. Need to make use of post_filter_command to inject a separate, case-sensitive grep into the pipeline.

Bonus: Added test for highlighting of listpri command.

todo.sh

@@ -1016,7 +1016,18 @@ case $action in
 "listpri" | "lsp" )
     shift ## was "listpri", new $1 is priority to list or first TERM
 
-    pri=$(printf "%s\n" "$1" | tr 'a-z' 'A-Z' | grep '^[A-Z]$') && shift || pri="[A-Z]"
+    if [ "${1:-}" ]
+    then
+        ## A priority was specified
+        pri=$( printf "%s\n" "$1" | tr 'a-z' 'A-Z' | grep '^[A-Z]$' ) || {
+            die "usage: $TODO_SH listpri PRIORITY
+note: PRIORITY must a single letter from A to Z."
+        }
+    else
+        ## No priority specified; show all priority tasks
+        pri="[A-Z]"
+    fi
+
     post_filter_command="grep '^ *[0-9]\+ (${pri}) '"
     _list "$TODO_FILE" "$@"
     ;;