The exercise talks of "characters" ("if that character appears only once") but treats the characters 'P' and 'p' as if they were the same. Badly written details.

Confusing instructions. It says
"-[: if the byte at the data pointer is zero, then instead of moving the instruction pointer forward to the next command, jump it forward to the command after the matching ] command."
and
"-]: if the byte at the data pointer is nonzero, then instead of moving the instruction pointer forward to the next command, jump it back to the command after the matching [ command."
but really, you have to move the instruction pointer to the matching "["/"]" command. Had to use a debugger to realize this.