Block Supports: Use Tag Processor for adding class-name to wrapper

Porting part of WordPress/gutenberg#46625

Replace use of fragile `preg_match` with Tag Processor when adding
an element class name to its wrapper.

src/wp-includes/block-supports/elements.php

@@ -56,29 +56,14 @@ function wp_render_elements_support( $block_content, $block ) {
 		return $block_content;
 	}
 
-	$class_name = wp_get_elements_class_name( $block );
-
 	// Like the layout hook this assumes the hook only applies to blocks with a single wrapper.
-	// Retrieve the opening tag of the first HTML element.
-	$html_element_matches = array();
-	preg_match( '/<[^>]+>/', $block_content, $html_element_matches, PREG_OFFSET_CAPTURE );
-	$first_element = $html_element_matches[0][0];
-	// If the first HTML element has a class attribute just add the new class
-	// as we do on layout and duotone.
-	if ( strpos( $first_element, 'class="' ) !== false ) {
-		$content = preg_replace(
-			'/' . preg_quote( 'class="', '/' ) . '/',
-			'class="' . $class_name . ' ',
-			$block_content,
-			1
-		);
-	} else {
-		// If the first HTML element has no class attribute we should inject the attribute before the attribute at the end.
-		$first_element_offset = $html_element_matches[0][1];
-		$content              = substr_replace( $block_content, ' class="' . $class_name . '"', $first_element_offset + strlen( $first_element ) - 1, 0 );
+	// Add the class name to the first element, presuming it's the wrapper, if it exists.
+	$tags = new WP_HTML_Tag_Processor( $block_content );
+	if ( $tags->next_tag() ) {
+		$tags->add_class( wp_get_elements_class_name( $block ) );
 	}
 
-	return $content;
+	return $tags->get_updated_html();
 }
 
 /**