On Wed, Aug 23, 2017 at 09:20:36PM +0000, Xufeng Liu wrote:
> Members of Routing Area Yang DT have had some discussions about the handling
> of various variants of regular expressions. The followings are the current
> state, and we are thinking that if this topic can be added to RFC6087bis:
>
> 1. Regular Expression Usage
> YANG uses regular expressions to restrict string values. Such a restriction
> can be a part of a "pattern" statement or a string matching function.
> [RFC7950] specifies that YANG regular expressions will conform to Appendix F
> in [XSD-TYPES].
> YANG models have been implemented in many different environments and the XSD
> variant of the regular expressions is not supported in many of these
> environments. There are currently more than a dozen popular regular
> expression variants implemented in various environments. While the usage of
> the XSD variant of regular expression described in [RFC7950] remains the
> preferred standard, a few conventions are prescribed to maximize the
> portability of YANG models between environments.
>
I strongly disagree with this statement. The standard format are XSD
regular expressions. RFC 7950 section 9.4.5:
The "pattern" statement, which is an optional substatement to the
"type" statement, takes as an argument a regular expression string,
as defined in [XSD-TYPES].
There is no notion of a 'preferred' standard.
> 1.1. Regular Expression Variant Choice Precedence
> YANG model designers SHOULD use the most portable syntax whenever possible.
> Under the condition that XSD compliance is satisfied and there are multiple
> choices for a given expression, the following precedence SHOULD be used to
> choose a regular expressions variant:
>
> o POSIX base
>
> o POSIX extended
>
> o BSD
>
> o GNU Regular Expression Extensions
>
> o C++ Regular Expressions with std::regex
>
> o Others
Strongly disagree. You either write YANG or something different. There
is no way to recognize what kind of regular expressions have been used
by the model designer. The value of a standard is that everybody does
the same.
> For example, either \d or [0-9] can be used with equivalent semantics and
> they are both compliant to [XSD-TYPES]. [0-9] is recommended because [0-9] is
> supported by POSIX base but \d is not.
>
> 1.2. Convention Guidelines
> 1.2.1. Avoid Character Category Escapes
> For example, in XSD regular expression, \d is a Character Category Escape
> denoting the range of digits, i.e., [0-9]. To maximize portability, the
> model designers SHOULD use [0-9] instead of \d.
>
> 1.2.2. Avoid Unicode Characters
> Unicode characters are allowed in XSD regular expressions, but are not
> supported in the POSIX variant. If possible, the model designers SHOULD avoid
> using Unicode characters, such as: \p{L} and \p{N}.
>
> 1.3. Conversion Tools
> Tools can automatically convert regular expressions from one variant to
> another. When a YANG model is implemented in an environment where XSD regular
> expressions are not supported, the recommended approach is to use a
> conversion tool. For example, if needed, anchor position characters, i.e.,
> '^' and '$', can be added by a regular expression conversion tool.
If conversion tools exist that can convert, then by all means use XSD
in the YANG model and use tools to convert to whatever format your
implementation prefers to use.
/js
--
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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