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.
