re module non-greedy matches broken

[lothar]

re:
4.2.1 Regular Expression Syntax
http://docs.python.org/lib/re-syntax.html

*?, +?, ??
Adding "?" after the qualifier makes it perform the match in non-greedy or
minimal fashion; as few characters as possible will be matched.

the regular expression module fails to perform non-greedy matches as
described in the documentation: more than "as few characters as possible"
are matched.

this is a bug and it needs to be fixed.

examples follow.

lothar@erda /ntd/vl
$ cat vwre.py
#! /usr/bin/env python

import re

vwre = re.compile("V.* ?W")
vwlre = re.compile("V.* ?WL")

if __name__ == "__main__":

newdoc = "V1WVVV2WWW "
vwli = re.findall(vwre , newdoc)
print "vwli[], expect", ['V1W', 'V2W']
print "vwli[], return", vwli

newdoc = "V1WLV2WV3WV4WL V5WV6WL"
vwlli = re.findall(vwlr e, newdoc)
print "vwlli[], expect", ['V1WL', 'V4WL', 'V6WL']
print "vwlli[], return", vwlli

lothar@erda /ntd/vl
$ python vwre.py
vwli[], expect ['V1W', 'V2W']
vwli[], return ['V1W', 'VVV2W']
vwlli[], expect ['V1WL', 'V4WL', 'V6WL']
vwlli[], return ['V1WL', 'V2WV3WV4WL', 'V5WV6WL']

lothar@erda /ntd/vl
$ python -V
Python 2.3.3

[Fredrik Lundh]

"lothar" wrote:

with respect to the documentation, the module is broken.
nope.
the module does not necessarily deliver a "minimal length" match for a
non-greedy pattern.

it isn't supposed to: a regular expression describes a *set* of matching
strings, and the engine is free to return any string from that set. Python's
engine returns the *first* string it finds that belongs to the set. if you use
a non-greedy operator, the engine will return the first non-greedy match
it finds, not the overall shortest non-greedy match.

if you don't want to understand how regular expressions work, don't use
them.

</F>

[lothar]

a non-greedy match is implicitly defined in the documentation to be one such
that there is no proper substring in the return which could also match the
regex.

the documentation implies the module will return a non-greedy match.

the module does not return a non-greedy match.
"Fredrik Lundh" <fr*****@python ware.com> wrote in message
news:ma******** *************** *************** @python.org...

"lothar" wrote:

with respect to the documentation, the module is broken.
nope.

the module does not necessarily deliver a "minimal length" match for a
non-greedy pattern.

it isn't supposed to: a regular expression describes a *set* of matching
strings, and the engine is free to return any string from that set.

Python's engine returns the *first* string it finds that belongs to the set. if you use a non-greedy operator, the engine will return the first non-greedy match
it finds, not the overall shortest non-greedy match.

if you don't want to understand how regular expressions work, don't use
them.

</F>

[Fredrik Lundh]

"lothar" wrote:

a non-greedy match is implicitly defined in the documentation to be one such
that there is no proper substring in the return which could also match the
regex.
no, that's not what it says. this is what is says:

Adding "?" after the qualifier makes it perform the match in non-greedy
or minimal fashion; as few characters as possible will be matched.

note that it says "qualifier" (that is, the preceeding *, +, or ? operator). it
doesn't say that the *entire* regex should be non-greedy. it does not say
that search, findall, sub etc. should look for the shortest possible overall
match. all it says is that the preceeding operator, and that operator only,
should look for the shortest possible match, rather than the longest.
the module does not return a non-greedy match.

it does. the problem is all in your head.

</F>