regular expressions questions

[Andrew M. Kuchling]

"Darrell" <darrell at dorb.com> writes:
> [Derek Thomson]
> > [ And now, at last, Perl 5.6 allows regexes to refer to other regexes. So
> >now we
> > can uses regexes to parse balanced expressions like matching parenthesis.

Actually, this was an experimental feature in Perl5.005.  Search
the perlre documentation for the (?{ code }) construct.

> >No doubt this will appear in Python's re module before long. ]

This is highly unlikely, because last time around we couldn't come up
with a way to do this that wasn't impressively ugly. See the thread
starting with this July 1998 article:
<URL:http://x28.deja.com/getdoc.xp?AN=372884721>

The general tenor of opinion seemed to be that, if you want to parse
expressions, use a real parser generator, and we certainly have enough
of those for Python.  Adding a feature to regexes for this produces
difficult-to-read code -- you need a pretty intimate knowledge of
exactly how the computer tries matches and then backtracks -- and
makes the matching engine more complicated.  So this feature is almost
certainly not going to be added.  (Elegant patches to add a new
feature, of course, can often reverse theoretical objections.)

-- 
A.M. Kuchling			http://starship.python.net/crew/amk/
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