Fix regex pattern when caret and dollar sign exists in the pattern

[jemershaw]

No description provided.

[jemershaw]

@llhotka -- Do you mind reviewing this PR?

[llhotka]

This is not correct. YANG uses the XSD flavour of regular expressions in which the characters ^ and $ keep their literal meaning. So, for example,

pattern '$';

matches the string $.

Correction: The ^ character is in fact special in XSD regex patterns and must be escaped to have the literal meaning. This doesn't change the argument though.

[jemershaw]

@llhotka I understand ^ and $ are their literal meaning. Please see https://github.com/openconfig/public/blob/master/release/models/types/openconfig-inet-types.yang#L81 which this is currently trying to solve since we don't want to include the ^ and $ meaning in the pattern.

[llhotka]

Yangson is based on IETF standards, OpenConfig folks often don't care too much about them. In this case, I believe they use another flavour of regular expressions, so there may possibly be other differences apart from the ^ and $ characters. So I suggest that you open an issue in the openconfig project.

[dbarrosop]

@llhotka Would you consider a PR that let's yangson choose between XSD style regexes (the default) and posix? Something like:

dm = DataModel.from_file('yang-library-ex2.json', [".", "../../../yang-modules/ietf"], regex_style={datamodel.PosixRegex, datamodel.XSDRegex})

Defaulting to XSDRegex, of course, to avoid changing the current behavior unless told explicitly.

I understand it's all Openconfig's fault but not supporting the best known repository of yang models feels like a big loss. Also, PosixRegex is better understood and more widely used.

[llhotka]

@dbarrosop This would make sense if the regex flavour is somehow declared in the YANG module, because otherwise it would be difficult to combine YANG modules using different flavours. I started a thread on this in the NETMOD mailing list.

[dbarrosop]

Yeah, I agree, ideally openconfig wouldn't break the standard... I am going to evaluate forking openconfig and fixing the regexes, if that turns out to be a reasonable alternative I will go that way, otherwise, I will show up here again to see if there is something we can do to fix this in the short-term. I am a huge fan of yangson and jetconf and wouldn't want to have to look for alternatives because of Openconfig... :(

Awesome work by the way, huge fan of yangson and jetconf as I said :)