Fix Python – Named regular expression group “(?Pregexp)”: what does “P” stand for?

In Python, the (?P…) syntax allows one to refer to the matched string through its name:
>>> import re
>>> match =‘(?P.*) (?P.*)’, ‘John 123456’)

What does “P” stand for? I could not find any hint in the official documentation.
I would love to get ideas about how to help my studen….