Block Style Variations: Simplify block style variation selector regex

The existing regex's first capture group is effectively dead. It only matched a literal `:(...)`, which isn't valid selector syntax (functional pseudo-classes like `:is()` and `:where()` always have a name between the colon and the parenthesis), so `$matches[1]` is always empty.

The second group did all the work, so dropping the dead group gives identical output and is much easier to understand.  

Props aaronrobertshaw, andrewserong, ramonopoly, wildworks. 

Fixes #65588.



git-svn-id: https://develop.svn.wordpress.org/trunk@62700 602fd350-edb4-49c9-b593-d223f7449a82

Author: ramonjd

src/wp-includes/class-wp-theme-json.php

@@ -5662,11 +5662,22 @@ protected static function get_block_style_variation_selector( $variation_name, $
 		$selector_parts = static::split_selector_list( $block_selector );
 		$result         = array();
 
+		/*
+		 * Append the variation class to each selector's ancestor: the first
+		 * run of characters before any combinator (whitespace) or pseudo-class
+		 * (`:`). Only the first match is replaced.
+		 *
+		 * Examples ("custom" variation):
+		 * - `.wp-block`              => `.wp-block.is-style-custom`
+		 * - `.wp-block .inner`       => `.wp-block.is-style-custom .inner`
+		 * - `.wp-block:where(.a .b)` => `.wp-block.is-style-custom:where(.a .b)`
+		 * - `:where(.outer .inner)`  => `:where(.outer.is-style-custom .inner)`
+		 */
 		foreach ( $selector_parts as $part ) {
 			$result[] = preg_replace_callback(
-				'/((?::\([^)]+\))?\s*)([^\s:]+)/',
+				'/[^\s:]+/',
 				function ( $matches ) use ( $variation_class ) {
-					return $matches[1] . $matches[2] . $variation_class;
+					return $matches[0] . $variation_class;
 				},
 				$part,
 				$limit