In Python, the (?P<group_name>…) syntax allows one to refer to the matched string through its name:
>>> import re
>>> match = re.search('(?P<name>.*) (?P<phone>.*)', 'John 123456')
>>> match.group('name')
'John'
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 students remember this syntax. Knowing what "P" does stand for (or might stand for) would be useful.
ベストアンサー1
Since we're all guessing, I might as well give mine: I've always thought it stood for Python. That may sound pretty stupid -- what, P for Python?! -- but in my defense, I vaguely remembered this thread [emphasis mine]:
Subject: Claiming (?P...) regex syntax extensions
From: Guido van Rossum ([email protected])
Date: Dec 10, 1997 3:36:19 pm
I have an unusual request for the Perl developers (those that develop the Perl language). I hope this (perl5-porters) is the right list. I am cc'ing the Python string-sig because it is the origin of most of the work I'm discussing here.
You are probably aware of Python. I am Python's creator; I am planning to release a next "major" version, Python 1.5, by the end of this year. I hope that Python and Perl can co-exist in years to come; cross-pollination can be good for both languages. (I believe Larry had a good look at Python when he added objects to Perl 5; O'Reilly publishes books about both languages.)
As you may know, Python 1.5 adds a new regular expression module that more closely matches Perl's syntax. We've tried to be as close to the Perl syntax as possible within Python's syntax. However, the regex syntax has some Python-specific extensions, which all begin with (?P . Currently there are two of them:
(?P<foo>...)Similar to regular grouping parentheses, but the text
matched by the group is accessible after the match has been performed, via the symbolic group name "foo".
(?P=foo)Matches the same string as that matched by the group named "foo". Equivalent to \1, \2, etc. except that the group is referred
to by name, not number.I hope that this Python-specific extension won't conflict with any future Perl extensions to the Perl regex syntax. If you have plans to use (?P, please let us know as soon as possible so we can resolve the conflict. Otherwise, it would be nice if the (?P syntax could be permanently reserved for Python-specific syntax extensions. (Is there some kind of registry of extensions?)
to which Larry Wall replied:
[...] There's no registry as of now--yours is the first request from outside perl5-porters, so it's a pretty low-bandwidth activity. (Sorry it was even lower last week--I was off in New York at Internet World.)
とにかく、私としては、あなたが「P」を祝福して使用することは間違いないと思います。(明らかに、この時点では Perl には「P」は必要ありません。:-) [...]
それで、P が最初に選ばれた理由が何だったのかはわかりません -- パターン? プレースホルダー? ペンギン? -- しかし、私がいつもそれを Python と関連付けてきた理由は理解できるでしょう。 (1) 私は正規表現が好きではなく、可能な限り避けていること、 (2) このスレッドが 15 年前に起こったことを考えると、ちょっと奇妙です。