Block Serialization Default Parser: Use PCRE v6.x-compatible subpattern syntax

[aduth]

Closes #13310

This pull request seeks to use a PCRE 6.x-compatible subpattern syntax in the block serialization parser. See #13310 for more information for the rationale of this change.

Testing instructions:

The affected PHP code here does not override the behavior of the WP_Block_Parser_Block class defined in WordPress 5.x . Therefore, you must test either by using an older version of WordPress, or by manually commenting the following line in your WordPress installation (see also #11015):

https://github.com/WordPress/WordPress/blob/b7897c576141adee3686056aff147ea09566c918/wp-settings.php#L258

...and applying a patch update to your local Gutenberg plugin to force the load of the bundled parser class definition:

diff --git a/gutenberg.php b/gutenberg.php
index 2df698bd4..f62a78bab 100644
--- a/gutenberg.php
+++ b/gutenberg.php
@@ -567,3 +567,5 @@ function gutenberg_add_responsive_body_class( $classes ) {
 }
 
 add_filter( 'body_class', 'gutenberg_add_responsive_body_class' );
+
+require_once dirname( __FILE__ ) . '/packages/block-serialization-default-parser/parser.php';

Then, confirm there are no regressions in the behavior of the parser. For example, a dynamic block such as the Latest Posts block should display in a front-end preview.

Ensure parser tests pass:

npm run test-unit-php

[pento]

👍🏻

[mcsf]

LGTM!