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

by

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 (gui…@CNRI.Reston.Va.US)

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.)

Anyway, as far as I’m concerned, you may certainly have ‘P’ with my
blessing. (Obviously Perl doesn’t need the ‘P’ at this point. 🙂 […]

So I don’t know what the original choice of P was motivated by — pattern? placeholder? penguins? — but you can understand why I’ve always associated it with Python. Which considering that (1) I don’t like regular expressions and avoid them wherever possible, and (2) this thread happened fifteen years ago, is kind of odd.

More Related Contents:

  1. python re.sub group: number after \number
  2. Python Regex instantly replace groups
  3. python regex: duplicate names in named groups
  4. Take first some character of string but if particular character will come consider that character also
  5. How to extract numbers from a string in Python?
  6. Is it worth using Python’s re.compile?
  7. Extract part of a regex match
  8. Regular expression matching a multiline block of text
  9. What is the difference between re.search and re.match?
  10. How can I do multiple substitutions using regex?
  11. regex error – nothing to repeat
  12. How to grab number after word in python
  13. Extract a string between double quotes
  14. Find the indexes of all regex matches?
  15. RegEx Get string between two strings that has line breaks
  16. Python – re.findall returns unwanted result
  17. Why Does a Repeated Capture Group Return these Strings?
  18. Splitting out the output of ps using Python
  19. How to use regex to find all overlapping matches
  20. How can a recursive regexp be implemented in python?
  21. Python regular expression pattern * is not working as expected
  22. Split string at every position where an upper-case word starts
  23. FutureWarning: The default value of regex will change from True to False in a future version
  24. Why does this take so long to match? Is it a bug?
  25. Regex matching – why does this not match and return None?
  26. How to use str.contains() with multiple expressions in pandas dataframes
  27. match dates using python regular expressions
  28. How to Fix JSON Key Values without double-quotes?
  29. python regular expression across multiple lines
  30. How to use regex with optional characters in python?

Leave a Comment