Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!newsfeed.eweka.nl!eweka.nl!feeder3.eweka.nl!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.011 X-Spam-Evidence: '*H*': 0.98; '*S*': 0.00; 'mrab': 0.04; 'python': 0.08; 'failing': 0.09; '"should': 0.16; 'abort': 0.16; 'assertion': 0.16; 'benefit.': 0.16; 'optimised': 0.16; 'repetition': 0.16; 'skip:\xc2 30': 0.16; 'subject:expression': 0.16; 'subject:regular': 0.16; 'wording': 0.16; 'meant': 0.17; 'wrote:': 0.18; '>>>': 0.18; 'instance': 0.18; 'repeated': 0.18; 'jan': 0.19; "aren't": 0.21; "doesn't": 0.22; 'header:In-Reply- To:1': 0.22; 'etc,': 0.23; 'url:mailman': 0.28; 'message- id:@mail.gmail.com': 0.28; 'pm,': 0.29; 'equivalent': 0.31; 'error.': 0.32; 'tue,': 0.32; 'url:listinfo': 0.32; 'instead': 0.33; 'received:209.85.160': 0.33; 'match': 0.34; 'to:addr:python- list': 0.34; 'it.': 0.34; 'received:209.85.160.46': 0.35; 'received:mail-pw0-f46.google.com': 0.35; 'url:python': 0.36; 'but': 0.37; 'run': 0.37; 'received:google.com': 0.37; 'could': 0.37; 'enough': 0.38; 'received:209.85': 0.38; 'put': 0.38; 'url:org': 0.39; 'why': 0.39; 'received:209': 0.40; 'to:addr:python.org': 0.40; 'worth': 0.61; 'cost': 0.63; 'believe': 0.65; '2012': 0.67; 'buy': 0.68; 'balanced': 0.84; 'optimisation': 0.84; 'cost,': 0.91 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :content-type:content-transfer-encoding; bh=6IRbnniILkKzhrdDdN0NK27891YP+LbU5BoTJk4o1I4=; b=MpXmn/OZLDUsRimgBAFwKe2MV5iBQoFPleCF61dKrmJfUYLEJm9IerJvbfmch3Igrl LEtrD1ICBdy4YZNSVxXi/VV19kulFk/SXgjs/BLrth5RtgpgsfQw8GibyQxkdY9Ksf2p DVEi2ozAFf8gxbP7riJOomnTWS9SOvtQsZ79w= MIME-Version: 1.0 In-Reply-To: <4F034FA3.6020902@mrabarnett.plus.com> References: <4f02c069$0$690$426a74cc@news.free.fr> <4F034FA3.6020902@mrabarnett.plus.com> From: Devin Jeanpierre Date: Tue, 3 Jan 2012 14:36:00 -0500 Subject: Re: Repeating assertions in regular expression To: python-list@python.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 49 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1325619405 news.xs4all.nl 6946 [2001:888:2000:d::a6]:50797 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:18437 > Put simply, it doesn't occur often enough to be worth it. The cost > outweighs the potential benefit. I don't buy it. You could backtrack instead of failing for \b+ and \b*, and it would be almost as fast as this optimization. -- Devin On Tue, Jan 3, 2012 at 1:57 PM, MRAB wrote: > On 03/01/2012 09:45, Devin Jeanpierre wrote: >>> >>> =C2=A0\\b\\b and \\b{2} aren't equivalent ? >> >> >> This sounds suspiciously like a bug! >> >>> =C2=A0Why the wording is "should never" ? Repeating a zero-width assert= ion is >>> not >>> =C2=A0forbidden, for instance : >>> >>>>>> =C2=A0import re >>>>>> =C2=A0re.compile("\\b\\b\w+\\b\\b") >>> >>> =C2=A0<_sre.SRE_Pattern object at 0xb7831140> >>>>>> >>>>>> >> >> I believe this is meant to refer to arbitrary-length repetitions, such >> as r'\b*', not simple concatenations like that. r'\b*' will abort the >> whole match if is run on a boundary, because Python detects a >> repetition of a zero-width match and decides this is an error. >> > r"\b+" can be optimised to r"\b", but r"\b*" can be optimised to r"". > r"\b\b", r"\b\b\b", etc, can be optimised to r"\b". > > So why doesn't it optimised? > > Because every potential optimisation has a cost, which is the time it > would take to look for it. > > That cost needs to be balanced against the potential benefit. > > How often do you see repeated r"\b"? > > Put simply, it doesn't occur often enough to be worth it. The cost > outweighs the potential benefit. > -- > http://mail.python.org/mailman/listinfo/python-list