Check support for /u flag in installed PCRE library

[jaswrks]

Issue Overview

Gutenberg's PEG parser expects the PCRE library to support the /u flag. We should probably check the installed PCRE library to confirm it supports this flag before using it. I opened a trac ticket in core to suggest a function for doing so: https://core.trac.wordpress.org/ticket/43224, and I've asked about this in the phpegjs package: nylen/phpegjs#9

Next Actions

• Follow-up on this issue in the phpegjs package.
• Follow-up on trac ticket #43224.

[nylen]

This case is already handled in the Gutenberg code. See: #1869, https://github.com/WordPress/gutenberg/search?q=_gutenberg_utf8_split

The remaining follow-up item here is a core merge task, for which I'd suggest creating a separate issue. WordPress' _mb_substr compatibility function is almost exactly the same as _gutenberg_utf8_split introduced for this purpose, so these functions should be refactored to share most of their logic and only split off where they need to be different.

[youknowriad]

I don't think we're using PEG in php anymore.