RFR: 8159337: Introduce a method in Locale class to return the language tags as per RFC 5646 convention [v6]

[Naoto Sato]

On Wed, 3 May 2023 20:32:25 GMT, Justin Lu <jlu at openjdk.org> wrote:

>> Please review this PR which adds the method `caseFoldLanguageTag(String languageTag)` to java.util.Locale.
>> 
>> This method case folds a language tag to adhere to _[section 2.1.1. Formatting of Language Tags of RFC5646](https://www.rfc-editor.org/rfc/rfc5646.html#section-2.1)_. This format is defined as _"All subtags, including extension and private use subtags, use lowercase letters with two exceptions: two-letter and four-letter subtags that neither appear at the start of the tag nor occur after singletons.  Such two-letter subtags are all uppercase ... and four-letter subtags are titlecase."_.
>> 
>> 
>> In order to match the behavior of existing language tag related Locale methods, this method matches the 2.1.1 RFC5646 specification with the following **exceptions**:
>> - Will not case fold variant subtags
>> - Will not case fold private use subtags prefixed by "lvariant"
>> 
>> As an example, `caseFoldLanguageTag("ja-kana-jp-x-lvariant-Oracle-JDK-Standard-Edition")` returns _"ja-Kana-JP-x-lvariant-Oracle-JDK-Standard-Edition"_. Further examples can be seen in the test file.
>
> Justin Lu has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Review: SBldr should be pre allocated with input tag
>  - Review: drop wwildcard import and replace subtags[i] with subtag
>  - Review: Include multiple singleton extensions in test

Marked as reviewed by naoto (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/13679#pullrequestreview-1413649585