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.

tests/t1250-listpri.sh

@@ -0,0 +1,71 @@
+#!/bin/sh
+
+test_description='list priority functionality
+'
+. ./test-lib.sh
+
+test_todo_session 'listpri usage' <<EOF
+>>> todo.sh listpri ?
+usage: todo.sh listpri PRIORITY
+note: PRIORITY must a single letter from A to Z.
+=== 1
+EOF
+
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(C) notice the sunflowers
+stop
+EOF
+test_todo_session 'basic listpri' <<EOF
+>>> todo.sh listpri A
+--
+TODO: 0 of 3 tasks shown
+
+>>> todo.sh -p listpri c
+2 (C) notice the sunflowers
+--
+TODO: 1 of 3 tasks shown
+EOF
+
+test_todo_session 'listpri highlighting' <<EOF
+>>> todo.sh listpri
+1 (B) smell the uppercase Roses +flowers @outside
+2 (C) notice the sunflowers
+--
+TODO: 2 of 3 tasks shown
+EOF
+
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(C) notice the sunflowers
+(m)others will notice this
+(n) not a prioritized task
+notice the (C)opyright
+EOF
+test_todo_session 'listpri filtering' <<EOF
+>>> todo.sh -p listpri
+1 (B) smell the uppercase Roses +flowers @outside
+2 (C) notice the sunflowers
+--
+TODO: 2 of 5 tasks shown
+
+>>> todo.sh -p listpri b
+1 (B) smell the uppercase Roses +flowers @outside
+--
+TODO: 1 of 5 tasks shown
+
+>>> todo.sh -p listpri c
+2 (C) notice the sunflowers
+--
+TODO: 1 of 5 tasks shown
+
+>>> todo.sh -p listpri m
+--
+TODO: 0 of 5 tasks shown
+
+>>> todo.sh -p listpri n
+--
+TODO: 0 of 5 tasks shown
+EOF
+
+test_done

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 "$TODO_FILE" "$@"
     ;;