UAX44: loose matching of symbolic values and the `is` prefix

[Mathias Bynens]

> On 7 Jun 2016, at 00:39, Nova Patch <patch.nova at gmail.com> wrote:
> 
> […] Based on my past research for Unicode Regular Expression Engines at IUC38, I suspect that there might not be any regex engine that actually supports syntax like Script=IsGreek as described in UAX44-LM3! If anybody knows otherwise, I’d love to hear about it.

This seems like a cut-and-dried case of reality not matching the specification, which is not helpful in any way. The sensible thing to do is to update the specification accordingly, as proposed.