[Patches] [ python-Patches-572936 ] (?(id/name)yes|no) re implementation

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Patches item #572936, was opened at 2002-06-24 03:41
Message generated for change (Comment added) made by loewis
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Category: Library (Lib)
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Gustavo Niemeyer (niemeyer)
>Assigned to: Gustavo Niemeyer (niemeyer)
Summary: (?(id/name)yes|no) re implementation

Initial Comment:
This patch implements a regular expression feature, which allows   
some interesting patterns, in the same way as implemented in perl.   
For example, (?(1)yes|no) matches with "yes" if group "1" exists, and   
with "no", if it doesn't. Without this feature, the regular expression   
must be duplicated to get the same results. In addition to perl's 
feature, it will also accept a Python named group as argument. 
   
Here's an example:   
   
(<)?\w+@\w+(\.\w+)+(?(1)>)   
  
This is a poor email matching regular expression, which will match   
with or without the "<>" symbols.   
   

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>Comment By: Martin v. Löwis (loewis)
Date: 2003-04-20 08:23

Message:
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I like the patch in principle, but I have a number of
additional concerns:
- there are no test cases
- the feature is declared experimental in perlre(1). Why?
- Shouldn't there be a semantic restriction that the back
  reference is only allowed if it points to a group that is
known
  to precede? I.e. is

  (X)|(?(1)Y)

  valid? If not, the restriction should be atleast documented, 
  but if possible, it should also be implemented.

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Comment By: Gustavo Niemeyer (niemeyer)
Date: 2003-04-19 23:30

Message:
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That patch is around for a long time. Should I work on it,
fixing that problem, and apply it? Do you agree with the
feature inclusion?

I remember that the main reason for implementing this is
because it is hard to achieve the same results without it.
You have to rewrite the whole match twice inside an or'ed group
(e.g. "(<... match email ...>|... match email ...)").


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Comment By: Martin v. Löwis (loewis)
Date: 2003-04-19 10:50

Message:
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If you add new opcodes, you should also change SRE_MAGIC.

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