Support Function Extensions

[hiltontj]

This issue is for tracking the support of JSONPath Function Extensions. There is a detailed outline of them in the spec here.

My interpretation of the spec is that there is a fixed and regulated registry of valid functions available to JSONPath; not that implementors of the spec can extend it as they please. Currently, there are only four registered functions:

• length
• count
• search
• match

Therefore, for initial function support in serde_json_path, I would like to provide support for those four functions, with the intention being to release support for any new function added to the registry in future, as they come.

It may be neat to allow for user-defined functions to extend serde_json_path, but I would like to keep that out of scope for now.

[hiltontj]

There are several ongoing issues in the IETF working group repo for JSON Path around the definition of the function extension point in the standard. I may hold off on implementing this until the details are finalized there.

[hiltontj]

Thought it worth mentioning that the standard for JSONPath makes use of I-Regexp which is described as, "a flavor of regular expressions that is limited in scope with the goal of interoperation across many different regular-expression libraries."

Furthermore, "I-Regexp does not provide advanced regular expression features such as capture groups, lookahead, or backreferences. It supports only a Boolean matching capability, i.e., testing whether a given regular expression matches a given piece of text."

Given that, it should be acceptable to use the regex crate to handle regular expressions, even though it supports a broader range of features than I-Regexp - those features just won't get used.