= \htmlentities( $img->getAttribute( 'src' ), ( \ENT_QUOTES | \ENT_SUBSTITUTE | \ENT_HTML401 ), $charset );
			$classes = $img->getAttribute( $attribute );

			$id = $this->extract_id_of_classes( $classes );

			$images[ $src ] = $id;
		}

		return $images;
	}

	/**
	 * Extracts image ID out of the image's classes.
	 *
	 * @param string $classes The classes assigned to the image.
	 *
	 * @return int The ID that's extracted from the classes.
	 */
	protected function extract_id_of_classes( $classes ) {
		if ( ! $classes ) {
			return 0;
		}

		/**
		 * Filter 'wpseo_extract_id_pattern' - Allows filtering the regex patern to be used to extract image IDs from class/attribute names.
		 *
		 * Defaults to the pattern that extracts image IDs from core's `wp-image-<ID>` native format in image classes.
		 *
		 * @api string The regex pattern to be used to extract image IDs from class names. Empty string if the whole class/attribute should be returned.
		 */
		$pattern = \apply_filters( 'wpseo_extract_id_pattern', '/(?<!\S)wp-image-(\d+)(?!\S)/i' );

		if ( $pattern === '' ) {
			return (int) $classes;
		}

		$matches = [];

		if ( \preg_match( $pattern, $classes, $matches ) ) {

			return (int) $matches[1];
		}

		return 0;
	}
}