Thanks to Peter Brett for pointing this out to me the other day. The
latest revision of the Unicode UTS#18
<https://www.unicode.org/reports/tr18/> covering regular expressions has
some significant changes. In particular, the level 3 specification
covering tailoring has been dropped.

The current version of the UTS is available at
https://www.unicode.org/reports/tr18/tr18-21.html.

Diff marks relative to the prior version can be viewed at
https://www.unicode.org/reports/tr18/tr18-20.html.

These changes appear to have been approved at UTC meeting #162 in May,
2020. Minutes are available at
https://www.unicode.org/L2/L2020/20015.htm; search for "UTS #18". The
relevant papers covering the changes appear to be:

  * L2/19-364R2
    <http://www.unicode.org/L2/L2019/19364r2-feedback-uts18.pdf>,
    Feedback on Proposed Update UTS #18, Markus Scherer, 2010-10-08.
  * L2/20-017 <http://www.unicode.org/L2/L2020/20017-pubrev.html>,
    Comments on Public Review Issues.
      o See the link to issue 404
        <https://www.unicode.org/review/pri404/feedback.html>.
  * L2/20-032
    <http://www.unicode.org/L2/L2020/20032-uts18-20-draft.pdf>, Document
    for review, Mark Davis and Andy Heninger, 2019-11-08.
  * L2/20-051
    <http://www.unicode.org/L2/L2020/20051-uts18-fdbk-response.pdf>,
    Response to feedback on UTS18, Mark Davis, 2020-01-14.

The upshot for us is that, when we get back around to looking at
std::regex replacements, we won't have to spend time worrying about
support for tailoring.

Tom.