I do not know how to ignore an item from the ER.
just need to get P1, but this returns /P1.
is possible to just ignore the bar?
$pattern = "#(/P[0-9])?#";
There are two options here:
Exclude it from the group, P1 will be the contents in the capture group:
$pattern = "#/(P[0-9])#";
Use a lookbehind so that the / isn't even a part of the match, the entire match will be P1:
$pattern = "#(?<=/)P[0-9]#";
Note that I also removed the ? after your group because I don't think you actually want it, this makes the previous element optional so the regex (/P[0-9])? would match literal any string (it would match an empty string if /P[0-9] could not be matched).
With preg_* functions, you can use the \K trick that reset the begin of the match, example:
$pattern = '~/\KP[0-9]~';