On 05/09/2017 17:35, Ladislav Lhotka wrote:
Robert Wilton píše v Po 04. 09. 2017 v 17:07 +0100:
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@j
acobs-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.
Yes, and the XSD RE language was also designed for pretty much the same purpose
(data type system).
But converting some parts of the XML RE syntax would probably be laborious:
Unicode support is of course hairy but since YANG permits it in the string type
it makes sense that the pattern language follow suit.
RE flavours used in modern programming languages support Unicode, so the
translation should be doable (if it hasn't been done yet).
Yes. POSIX extended regex (that one proposed by OpenConfig) is the odd
one out here because it doesn't support unicode.
Still I haven't seen any standards based RFC or IETF draft YANG models
that need to match either unicode properties or blocks. The IPv4/v6
zone address uses them, but I suspect that '\w' would have been sufficient.
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.
But now with the "invert-match" modifier in YANG 1.1, implementations have to be
able to perform such set differences anyway, right?
No. Character class subtraction applies to a single character class
match in the expression. The "invert-match" applies to the whole regex
check. The same regex check can be performed and the boolean result
reversed.
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.
I believe that tools intended for general use should follow the YANG spec
literally.
I don't fully agree. I think that they only need to cover the parts of
the YANG spec for the models that they are using (or might use). If
nobody uses Unicode blocks then it doesn't really matter whether a given
tool supports them or not. It is always possible to caveat and add
support for the missing bits later. E.g. if I was writing a bespoke
XPATH implementation for YANG then there is probably quite a lot of the
XPATH spec that I would also leave out as well, and just concentrate on
the parts that people actually use, or are likely to use.
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.
I think they should mean exactly what XSD spec says they mean.
Yes. I agree. I'm only listing that conversions are likely necessary
to convert an XSD RE into one of the other standard RE implementations.
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.
Yes, and this is a subtle point that could otherwise be easily overlooked.
3) The syntax for escaping characters seems to differ in XML RE from
other common languages.
4) Linebreak match handling seems to differ.
3 and 4 are IMO not a big deal.
But they do matter to avoid "Tools and libraries would then differ in
the degree of sloppiness and possibly give different results, which is
not good."
These conversions would need to be done regardless, but would seem to be
much quicker/simpler to implement than the ones above.
Tools and libraries would then differ in the degree of sloppiness and possibly
give different results, which is not good.
Sorry, I don't get this last point.
However, I have thrown in the towel on my regex crusade.
But I still suspect that most model authors/readers are likely to get
the usage of '\d' wrong ... but perhaps it doesn't really matter.
Thanks,
Rob
Lada
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/>
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