edit cover block regex for featured image WordPress#4

For the block layout see: https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/cover/edit/index.js#L327-L367
For the HTML tags capturing by class name see: https://github.com/WordPress/gutenberg/blob/51db4bf888e6b18cf9d18266108114d61070f3ad/lib/block-supports/layout.php#L232

packages/block-library/src/cover/index.php

@@ -62,12 +62,11 @@ class="wp-block-cover__image-background"
 
 		/* Inserts the featured image between the (1st) cover 'background' `span` and 'inner_container' `div`,
 		   and removes eventual withespace characters between the two (typically introduced at template level) */
-		$content = preg_replace(
-		    '/(<span\b[^>]*wp-block-cover__background[\s|"][^>]*>.*<\/span>)\s*(<div\b[^>]*wp-block-cover__inner-container[\s|"][^>]*>)/U',
-		    "$1$image$2",
-		    $content,
-		    1
-		);
+		$inner_container_start = /<div\b[^>]+wp-block-cover__inner-container[\s|"][^>]*>/U;
+		if ( 1 === preg_match( $inner_container_start, $content, $matches, PREG_OFFSET_CAPTURE ) {
+			list( $fullmatch, $offset ) = $matches[0];
+			$content = substr( $content, 0, $offset ) . $image_tag . substr( $content, $offset );
+		}
 
 	}