Re: [netmod] Potential additions to rfc6087bis: RegEx guidelines

Robert Wilton <rwilton@cisco.com> Mon, 04 September 2017 16:07 UTC

Return-Path: <rwilton@cisco.com>
X-Original-To: netmod@ietfa.amsl.com
Delivered-To: netmod@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 135D41321A5 for <netmod@ietfa.amsl.com>; Mon, 4 Sep 2017 09:07:45 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -14.501
X-Spam-Level:
X-Spam-Status: No, score=-14.501 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RCVD_IN_DNSWL_HI=-5, SPF_PASS=-0.001, USER_IN_DEF_DKIM_WL=-7.5] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (1024-bit key) header.d=cisco.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ndjV25_HVOKx for <netmod@ietfa.amsl.com>; Mon, 4 Sep 2017 09:07:42 -0700 (PDT)
Received: from aer-iport-2.cisco.com (aer-iport-2.cisco.com [173.38.203.52]) (using TLSv1.2 with cipher DHE-RSA-SEED-SHA (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id EBFFE1321C4 for <netmod@ietf.org>; Mon, 4 Sep 2017 09:07:41 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=cisco.com; i=@cisco.com; l=7271; q=dns/txt; s=iport; t=1504541262; x=1505750862; h=subject:to:references:from:message-id:date:mime-version: in-reply-to:content-transfer-encoding; bh=65CKN3/P8a+KtZJK1VaF5HzbIDKB6yw73ukKsuPoVXc=; b=Ks1XczGBodJWgHSAqSJDLNa/aVl2LmH1SRLliMZnXrQRrlUlj06qwTF2 uEIhdGM1iWdnSx6C83J8nbmWDVTDYBgBKjw+pWlz/5Nt/izH3H1WALlev Ow1eYX16fawstk8LYG8YfLUFCCcSgcAYim1ajzsqKz1UrQk1rDrTiImxW s=;
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: A0AWAgB/ea1Z/xbLJq1TBwMZAQEBAQEBAQEBAQEHAQEBAQGEPoEVg3eLFJEed5RLgngKGAuETE8ChGAUAQIBAQEBAQEBayiFGAEBAQECAQEBIQ8BBTYZAgkCEAgCAiYCAhsMMAYBDAYCAQEQB4oOCBCWH51mgieLUgEBAQEBAQEBAQEBAQEBAQEBAQEBAR0FgQiCHYNQgWMrgkg1hEpMJoJMgmEFoHSUUYtUhx2NV4Qcgw0DBgUCGYE5NiGBDTIhCBwVSYccPzYBixgBAQE
X-IronPort-AV: E=Sophos;i="5.41,475,1498521600"; d="scan'208";a="654393103"
Received: from aer-iport-nat.cisco.com (HELO aer-core-1.cisco.com) ([173.38.203.22]) by aer-iport-2.cisco.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 04 Sep 2017 16:07:39 +0000
Received: from [10.63.23.66] (dhcp-ensft1-uk-vla370-10-63-23-66.cisco.com [10.63.23.66]) by aer-core-1.cisco.com (8.14.5/8.14.5) with ESMTP id v84G7d7j018680; Mon, 4 Sep 2017 16:07:39 GMT
To: Ladislav Lhotka <lhotka@nic.cz>, netmod@ietf.org
References: <f7151a6b-9deb-52ad-62a9-78b29a552540@cisco.com> <20170830102902.2n5q6rgq2x2dxfq2@elstar.local> <e8482a9c-cba3-28e2-9ffa-ec5eb5c1c0a4@cisco.com> <20170830123156.cssrg5kklpo67fie@elstar.local> <CABCOCHTtN611FO2ov2kTLtZx-Q3=tzgH7Xk9uGvFUD1WuyMZyw@mail.gmail.com> <b13c5e9a-e9f9-96e9-8823-0402fb74af09@cisco.com> <1504223854014.55228@Aviatnet.com> <847e5bf9-7b3d-9ff8-9954-970f32a2094c@cisco.com> <20170902073342.xoziwor4tdr5bipw@elstar.local> <D5D00209.C5C67%acee@cisco.com> <20170902112832.ymorfgdthobeio6q@elstar.local> <CABCOCHTC2MhBu0Zu44Z=f+J04HiENjQR+J0Sxy-arjcDmBHb_A@mail.gmail.com> <1e95ba5d-7aa2-e08f-56f9-27aa70822a11@cisco.com> <1504537140.5874.38.camel@nic.cz>
From: Robert Wilton <rwilton@cisco.com>
Message-ID: <f0ddf7bd-c249-389f-e34b-0b901697307e@cisco.com>
Date: Mon, 04 Sep 2017 17:07:39 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0
MIME-Version: 1.0
In-Reply-To: <1504537140.5874.38.camel@nic.cz>
Content-Type: text/plain; charset="utf-8"; format="flowed"
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Archived-At: <https://mailarchive.ietf.org/arch/msg/netmod/aDBZ1LXf7EeBXwJ5qm377YG3HMo>
Subject: Re: [netmod] Potential additions to rfc6087bis: RegEx guidelines
X-BeenThere: netmod@ietf.org
X-Mailman-Version: 2.1.22
Precedence: list
List-Id: NETMOD WG list <netmod.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/netmod>, <mailto:netmod-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/netmod/>
List-Post: <mailto:netmod@ietf.org>
List-Help: <mailto:netmod-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/netmod>, <mailto:netmod-request@ietf.org?subject=subscribe>
X-List-Received-Date: Mon, 04 Sep 2017 16:07:45 -0000

Hi Lada,

On 04/09/2017 15:59, Ladislav Lhotka wrote:
> Robert Wilton píše v Po 04. 09. 2017 v 15:05 +0100:
>> Hi Andy,
>>
>> On 02/09/2017 17:46, Andy Bierman wrote:
>>> On Sat, Sep 2, 2017 at 4:28 AM, Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> wrote:
>>>> On Sat, Sep 02, 2017 at 10:39:57AM +0000, Acee Lindem (acee) wrote:
>>>>> This is not an effort to change or bifurcate the YANG 1.1. It is simply to
>>>>> RECOMMEND a proper subset of XSD pattern that is more portable.
>>>>>
>>>> If you implement YANG as it is defined, pattern are portable. Given
>>>> this, I do not understand the notion of 'more portable'.
>>>>
>>>> Anyway, it seems that those who want a more portable subset do not
>>>> even agree on what that subset is. Perhaps people pushing for this
>>>> should go and write an I-D that explains why a 'more portable' subset
>>>> is needed (which problems are we fixing), that defines such a 'more
>>>> portable subset', and which includes the reasoning how the subset has
>>>> been determined.
>>>>
>>>>
>>>
>>> I do not agree that the YANG pattern contains a string that is both a POSIX and XSD regular expression.
>>> The RFC is very clear it contains an XSD expression. Pretending it is both is a hack that does not even seem
>>> to work 100%, so it is not reliable.
>>   I am not suggesting that the YANG pattern is both a POSIX and XSD regular expression.
>>
>> I am only suggesting that the guidelines recommend that authors use a subset of XSD, to make it easier to programmatically *convert* the 'XSD subset compliant regular expression' into a functionally equivalent regular expression for whatever regular expression engine the tooling decides to use.
> And that's the point, I think: each developer needs to get a library function so
> as to translate the XSD pattern into a native regex of whatever programming
> language he/she is currently using. So I guess what we really need is to
> identify libraries for common languages that do it correctly - or write simple
> translators ourselves if none is available.
Yes, exactly.

Looking at http://www.regular-expressions.info/ then XML RE does look 
like a good standard choice of RE language for YANG pattern statements 
because it is generally one of the most basic RE languages, and hence it 
should be feasible to convert an XML RE into a form usable by most RE 
languages.

But converting some parts of the XML RE syntax would probably be laborious:
1) E.g. the unicode property '\p{Nd}' that is equivalent to '\d' matches 
590 characters 
(http://www.fileformat.info/info/unicode/category/Nd/list.htm). There 
are approx 32 unicode properties, presumably these could also be 
extended over time as well.
2) There are currently 105 unicode blocks, which each block is a 
discrete range of characters (e.g. \p{InTibetan}: U+0F00–U+0FFF)
3) Handling the character class subtraction is also possible, but 
probably tedious to implement, since it requires the translation to 
fully understand the set of characters in the character class so it can 
form an equivalent character class without any subtractions.
These were the three parts of the XML RE that I was hoping to discourage 
in the YANG author guidelines so that performing a translation is much 
easier.  Spotting these 3 parts in the regex should be simple, so the 
translation would still be robust, even if not complete.

There are other conversions that may also need to be performed 
(depending on the target RE engine):
1) Character class shorthands (e.g. \d, \w) need to be converted to 
represent the Unicode set equivalent, since for a lot of engines they 
only match ASCII characters.  For '\s' it must match ASCII whitespace only.
2) If the engine supports greedy alternation (e.g. POSIX basic/extended 
regex), then alternations need to be converted to an eager form if required.
3) The syntax for escaping characters seems to differ in XML RE from 
other common languages.
4) Linebreak match handling seems to differ.
These conversions would need to be done regardless, but would seem to be 
much quicker/simpler to implement than the ones above.

Thanks,
Rob


>
>> E.g. this seems to be the approach used by "libyang" that uses libpcre as the backend RE library rather than libxml.  Unfortunately, I think that the libyang library would currently fail if the pattern statement contained "[[A-Z]-[P-R]]" because it looks like the PCRE2 language does not support character class subtraction.  ACAICT, no standard YANG modules currently support character class subtraction, so the authors of libyang have a choice here:
> Note that your example is incorrect, it should be [A-Z-[P-R]]. FWIW, Python
> module PyXB (that I used in Yangson library) does support this.
>
> Lada
>
>>    (i) write a block of code that most likely nobody is going to use, or
>>    (ii) document the limitation, spot character class subtraction in the regex, and flag that it is not supported (or perhaps just ignore it).
>>
>>
>>> If the community wants to support both XSD and POSIX expressions, then the proper engineering
>>> solution is to introduce a new statement that is defined to contain a POSIX expression.
>>> This can be done with a YANG extension now and added to YANG 2.0 later.
>>   I think that this is an inferior solution:
>> - there are many languages that YANG tools could be written in: C/C++, Python, Java, Go, Rust, Javascript are all reasonably plausible choices.
>> - they all have similar, but with small differences regular expression flavours (according to http://www.regular-expressions.info/reference.html).
>> - Personally, I see no inherent advantage of the POSIX Extended Regex over XML RE.   In fact, given that it doesn't support Unicode at all, it would seem to be a somewhat strange choice for a second pattern statement.
>> - Nor does it seem pragmatic to introduce lots of different flavors of pattern statements into YANG each supporting a different regex syntax.
>>
>> I also don't like the solution that every YANG tool maker has to either link against libxml2,  or write their own efficient regular expression engine.  I'm not convinced that what the world needs is yet more regular expression implementations :-)
>>
>> So, I still see that the better technical solution is always only define the pattern statements in XML RE language, but to strongly encourage folks to use a subset of that language for standards models (which they appear to be doing anyway) to make it easier to covert the regular expression into compatible versions for other engines.
>>
>> Thanks,
>> Rob
>>
>>
>>>   
>>>> /js
>>>>
>>> Andy
>>>   
>>>> --
>>>> Juergen Schoenwaelder           Jacobs University Bremen gGmbH
>>>> Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
>>>> Fax:   +49 421 200 3103         <http://www.jacobs-university.de/>
>>>>
>>>> _______________________________________________
>>>> netmod mailing list
>>>> netmod@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>>
>>   
>> _______________________________________________
>> netmod mailing list
>> netmod@ietf.org
>> https://www.ietf.org/mailman/listinfo/netmod