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https://issues.apache.org/jira/browse/SOLR-8110?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15170698#comment-15170698
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Jack Krupansky commented on SOLR-8110:
--------------------------------------

Dollar sign is permitted in Java identifier, including at the start. As per the 
Java Spec, "The "Java letters" include uppercase and lowercase ASCII Latin 
letters A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical 
reasons, the ASCII underscore (_, or \u005f) and dollar sign ($, or \u0024)." 
It goes on to say that "The $ character should be used only in mechanically 
generated source code or, rarely, to access pre-existing names on legacy 
systems."

See:
https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.8

If anything, I had been assuming that we were proposing a superset of Java 
identifiers (hyphen, dot as part of name.)

I'm not positive whether there might be any conflict with parameter 
substitution for dollar sign.


> Start enforcing field naming recomendations in next X.0 release?
> ----------------------------------------------------------------
>
>                 Key: SOLR-8110
>                 URL: https://issues.apache.org/jira/browse/SOLR-8110
>             Project: Solr
>          Issue Type: Improvement
>            Reporter: Hoss Man
>         Attachments: SOLR-8110.patch, SOLR-8110.patch
>
>
> For a very long time now, Solr has made the following "recommendation" 
> regarding field naming conventions...
> bq. field names should consist of alphanumeric or underscore characters only 
> and not start with a digit.  This is not currently strictly enforced, but 
> other field names will not have first class support from all components and 
> back compatibility is not guaranteed.  ...
> I'm opening this issue to track discussion about if/how we should start 
> enforcing this as a rule instead (instead of just a "recommendation") in our 
> next/future X.0 (ie: major) release.
> The goals of doing so being:
> * simplify some existing code/apis that currently use hueristics to deal with 
> lists of field and produce strange errors when the huerstic fails (example: 
> ReturnFields.add)
> * reduce confusion/pain for new users who might start out unaware of the 
> recommended conventions and then only later encountering a situation where 
> their field names are not supported by some feature and get frustrated 
> because they have to change their schema, reindex, update index/query client 
> expectations, etc...



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