[issue15956] backreference to named group does not work

[Steve Newcomb]

Steve Newcomb added the comment:

I have re-read the documentation on re.sub().  Even now, now that I understand that the \g<groupname> syntax applies to the repl argument only, I cannot see how the documentation can be understood that way.  The paragraph in which the explanation of the \g<groupname> syntax appears does not mention the repl argument by name, and neither does the preceding paragraph. 

The paragraph before the preceding paragraph is about the pattern argument, not the repl argument, and it consists entirely of the words, "The pattern may be a string or an RE object." 

So I don't see how the explanation of the \g<groupname> syntax can be understood as applying only to the repl argument, even though you have now informed me that that's the case (which is helpful to know -- thanks!).  Indeed, the paragraph that explains the \g<groupname> syntax *still* appears to me to be discussing the pattern argument.  And it even mentions the <?P<name> syntax, which can only appear in a pattern, not in a repl, in the very same sentence as the \g<groupname> syntax, even though those two syntactic features appear in *different* expression languages, and no single expression language has both of them.  

So there is no clear indication that it is discussing two different expression languages.  Indeed, another syntactic feature, \groupnumber, also discussed in the same paragraph, *is* found in both expression languages, so it's even more confusing to a person who knows that both <?P<groupname> and \groupnumber appear in the pattern expression language.  There is nothing in the documentation that would inform a person (such as myself) that the \g<groupname> syntax is not also part of the pattern expression language, just as the other two features are.

(And why isn't \g<groupname> part of the pattern language, anyway, or at least some way to refer to a match made in a previous *named* group?  It would be very convenient to be able to do that, particularly when using a dynamically-created regexp to parse strings delimited with a choice of delimiters that must match at both ends.)

In other words, this documentation could be beneficially improved.

----------
resolution: invalid -> 
status: closed -> open

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15956>
_______________________________________