Regex to capture attachment ID from image tag markup.

attachment-id-cap.php

<?php
/**
 * Returns attachment ids from the string using regex.
 * match string "wp-image-{the-id-here}"
 * regex101 link: https://regex101.com/r/aacOaV/1/
 *
 * @param string $content
 * @return array array of attachment ids.
 */
public function get_attachment_ids_from_content( $content ) {
	$re = '/wp-image-([0-9]+)/mi';
	preg_match_all( $re, $content, $matches, PREG_SET_ORDER, 0 );

	$attachment_ids = [];
	foreach ( $matches as $match ) {
		if ( is_numeric( $match[1] ) ) {
			$attachment_ids[] = (int) $match[1];
		}
	}

	return $attachment_ids;
}