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

[Ladislav Lhotka <[e-mail]>]

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 <[e-mail]> 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.

> 
> 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: [phone]         Campus Ring 1 | 28759 Bremen | Germany
> > > Fax:   [phone]         <http://www.jacobs-university.de/>
> > > 
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>  
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Ladislav Lhotka
Head, CZ.NIC Labs
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