The Grapheme polyfill is failing tests when using \X as regex for clusters

[stof]

See CI jobs for PHP 7.3+ which are failing a test when using the polyfill:

$this->assertSame('☎', grapheme_substr('☢☎❄', 1, 1));

\X does not split those as separate grapheme clusters.

[stof]

Apparently, the \X is correct only starting in PCRE 10.44: PCRE2Project/pcre2#410

We might need to change our condition for the usage of the fallback regex.

[Ayesh]

Yeah, it's only fixed in PCRE 10.44 (I was the one who reported).
This is worked around in the proposed grapheme_str_split PR. I don't think there is a way to overcome this, apart from skipping the assertions on older PCRE2 versions.

[stof]

We already have some code dealing with that in

polyfill/src/Intl/Grapheme/Grapheme.php

Line 14 in a5ae132

\define('SYMFONY_GRAPHEME_CLUSTER_RX', ((float) \PCRE_VERSION < 10 ? (float) \PCRE_VERSION >= 8.32 : (float) \PCRE_VERSION >= 10.39) ? '\X' : Grapheme::GRAPHEME_CLUSTER_RX);