nt',
				__( 'Unable to retrieve body from response at this URL.' ),
				array( 'status' => WP_Http::NOT_FOUND )
			);
		}

		return $remote_body;
	}

	/**
	 * Parses the title tag contents from the provided HTML.
	 *
	 * @since 5.9.0
	 *
	 * @param string $html The HTML from the remote website at URL.
	 * @return string The title tag contents on success. Empty string if not found.
	 */
	private function get_title( $html ) {
		$pattern = '#<title[^>]*>(.*?)<\s*/\s*title>#is';
		preg_match( $pattern, $html, $match_title );

		if ( empty( $match_title[1] ) || ! is_string( $match_title[1] ) ) {
			return '';
		}

		$title = trim( $match_title[1] );

		return $this->prepare_metadata_for_output( $title );
	}

	/**
	 * Parses the site icon from the provided HTML.
	 *
	 * @since 5.9.0
	 *
	 * @param string $html The HTML from the remote website at URL.
	 * @param string $url  The target website URL.
	 * @return string The icon URI on success. Empty string if not found.
	 */
	private function get_icon( $html, $url ) {
		// Grab the icon's link element.
		$pattern = '#<link\s[^>]*rel=(?:[\"\']??)\s*(?:icon|shortcut icon|icon shortcut)\s*(?:[\"\']??)[^>]*\/?>#isU';
		preg_match( $pattern, $html, $element );
		if ( empty( $element[0] ) || ! is_string( $element[0] ) ) {
			return '';
		}
		$element = trim( $element[0] );

		// Get the icon's href value.
		$pattern = '#href=([\"\']??)([^\" >]*?)\\1[^>]*#isU';
		preg_match( $pattern, $element, $icon );
		if ( empty( $icon[2] ) || ! is_string( $icon[2] ) ) {
			return '';
		}
		$icon = trim( $icon[2] );

		// If the icon is a data URL, return it.
		$parsed_icon = parse_url( $icon );
		if ( isset( $parsed_icon['scheme'] ) && 'data' === $parsed_icon['scheme'] ) {
			return $icon;
		}

		// Attempt to convert relative URLs to absolute.
		if ( ! is_string( $url ) || '' === $url ) {
			return $icon;
		}
		$parsed_url = parse_url( $url );
		if ( isset( $parsed_url['scheme'] ) && isset( $parsed_url['host'] ) ) {
			$root_url = $parsed_url['scheme'] . '://' . $parsed_url['host'] . '/';
			$icon     = WP_Http::make_absolute_url( $icon, $root_url );
		}

		return $icon;
	}

	/**
	 * Parses the meta description from the provided HTML.
	 *
	 * @since 5.9.0
	 *
	 * @param array $meta_elements {
	 *     A multi-dimensional indexed array on success, else empty array.
	 *
	 *     @type string[] $0 Meta elements with a content attribute.
	 *     @type string[] $1 Content attribute's opening quotation mark.
	 *     @type string[] $2 Content attribute's value for each meta element.
	 * }
	 * @return string The meta description contents on success. Empty string if not found.
	 */
	private function get_description( $meta_elements ) {
		// Bail out if there are no meta elements.
		if ( empty( $meta_elements[0] ) ) {
			return '';
		}

		$description = $this->get_metadata_from_meta_element(
			$meta_elements,
			'name',
			'(?:description|og:description)'
		);

		// Bail out if description not found.
		if ( '' === $description ) {
			return '';
		}

		return $this->prepare_metadata_for_output( $description );
	}

	/**
	 * Parses the Open Graph (OG) Image from the provided HTML.
	 *
	 * See: https://ogp.me/.
	 *
	 * @since 5.9.0
	 *
	 * @param array  $meta_elements {
	 *     A multi-dimensional indexed array on success, else empty array.
	 *
	 *     @type string[] $0 Meta elements with a content attribute.
	 *     @type string[] $1 Content attribute's opening quotation mark.
	 *     @type string[] $2 Content attribute's value for each meta element.
	 * }
	 * @param string $url The target website URL.
	 * @return string The OG image on success. Empty string if not found.
	 */
	private function get_image( $meta_elements, $url ) {
		$image = $this->get_metadata_from_meta_element(
			$meta_elements,
			'property',
			'(?:og:image|og:image:url)'
		);

		// Bail out if image not found.
		if ( '' === $image ) {
			return '';
		}

		// Attempt to convert relative URLs to absolute.
		$parsed_url = parse_url( $url );
		if ( isset( $parsed_url['scheme'] ) && isset( $parsed_url['host'] ) ) {
			$root_url = $parsed_url['scheme'] . '://' . $parsed_url['host'] . '/';
			$image    = WP_Http::make_absolute_url( $image, $root_url );
		}

		return $image;
	}

	/**
	 * Prepares the metadata by:
	 *    - stripping all HTML tags and tag entities.
	 *    - converting non-tag entities into characters.
	 *
	 * @since 5.9.0
	 *
	 * @param string $metadata The metadata content to prepare.
	 * @return string The prepared metadata.
	 */
	private function prepare_metadata_for_output( $metadata ) {
		$metadata = html_entity_decode( $metadata, ENT_QUOTES, get_bloginfo( 'charset' ) );
		$metadata = wp_strip_all_tags( $metadata );
		return $metadata;
	}

	/**
	 * Utility function to build cache key for a given URL.
	 *
	 * @since 5.9.0
	 *
	 * @param string $url The URL for which to build a cache key.
	 * @return string The cache key.
	 */
	private function build_cache_key_for_url( $url ) {
		return 'g_url_details_response_' . md5( $url );
	}

	/**
	 * Utility function to retrieve a value from the cache at a given key.
	 *
	 * @since 5.9.0
	 *
	 * @param string $key The cache key.
	 * @return mixed The value from the cache.
	 */
	private function get_cache( $key ) {
		return get_site_transient( $key );
	}

	/**
	 * Utility function to cache a given data set at a given cache key.
	 *
	 * @since 5.9.0
	 *
	 * @param string $key  The cache key under which to store the value.
	 * @param string $data The data to be stored at the given cache key.
	 * @return bool True when transient set. False if not set.
	 */
	private function set_cache( $key, $data = '' ) {
		$ttl = HOUR_IN_SECONDS;

		/**
		 * Filters the cache expiration.
		 *
		 * Can be used to adjust the time until expiration in seconds for the cache
		 * of the data retrieved for the given URL.
		 *
		 * @since 5.9.0
		 *
		 * @param int $ttl The time until cache expiration in seconds.
		 */
		$cache_expiration = apply_filters( 'rest_url_details_cache_expiration', $ttl );

		return set_site_transient( $key, $data, $cache_expiration );
	}

	/**
	 * Retrieves the head element section.
	 *
	 * @since 5.9.0
	 *
	 * @param string $html The string of HTML to parse.
	 * @return string The `<head>..</head>` section on success. Given `$html` if not found.
	 */
	private function get_document_head( $html ) {
		$head_html = $html;

		// Find the opening `<head>` tag.
		$head_start = strpos( $html, '<head' );
		if ( false === $head_start ) {
			// Didn't find it. Return the original HTML.
			return $html;
		}

		// Find the closing `</head>` tag.
		$head_end = strpos( $head_html, '</head>' );
		if ( false === $head_end ) {
			// Didn't find it. Find the opening `<body>` tag.
			$head_end = strpos( $head_html, '<body' );

			// Didn't find it. Return the original HTML.
			if ( false === $head_end ) {
				return $html;
			}
		}

		// Extract the HTML from opening tag to the closing tag. Then add the closing tag.
		$head_html  = substr( $head_html, $head_start, $head_end );
		$head_html .= '</head>';

		return $head_html;
	}

	/**
	 * Gets all the meta tag elements that have a 'content' attribute.
	 *
	 * @since 5.9.0
	 *
	 * @param string $html The string of HTML to be parsed.
	 * @return array {
	 *     A multi-dimensional indexed array on success, else empty array.
	 *
	 *     @type string[] $0 Meta elements with a content attribute.
	 *     @type string[] $1 Content attribute's opening quotation mark.
	 *     @type string[] $2 Content attribute's value for each meta element.
	 * }
	 */
	private function get_meta_with_content_elements( $html ) {
		/*
		 * Parse all meta elements with a content attribute.
		 *
		 * Why first search for the content attribute rather than directly searching for name=description element?
		 * tl;dr The content attribute's value will be truncated when it contains a > symbol.
		 *
		 * The content attribute's value (i.e. the description to get) can have HTML in it and be well-formed as
		 * it's a string to the browser. Imagine what happens when attempting to match for the name=description
		 * first. Hmm, if a > or /> symbol is in the content attribute's value, then it terminates the match
		 * as the element's closing symbol. But wait, it's in the content attribute and is not the end of the
		 * element. This is a limitation of using regex. It can't determine "wait a minute this is inside of quotation".
		 * If this happens, what gets matched is not the entire element or all of the content.
		 *
		 * Why not search for the name=description and then content="(.*)"?
		 * The attribute order could be opposite. Plus, additional attributes may exist including being between
		 * the name and content attributes.
		 *
		 * Why not lookahead?
		 * Lookahead is not constrained to stay within the element. The first <meta it finds may not include
		 * the name or content, but rather could be from a different element downstream.
		 */
		$pattern = '#<meta\s' .

				/*
				 * Allows for additional attributes before the content attribute.
				 * Searches for anything other than > symbol.
				 */
				'[^>]*' .

				/*
				* Find the content attribute. When found, capture its value (.*).
				*
				* Allows for (a) single or double quotes and (b) whitespace in the value.
				*
				* Why capture the opening quotation mark, i.e. (["\']), and then backreference,
				* i.e \1, for the closing quotation mark?
				* To ensure the closing quotation mark matches the opening one. Why? Attribute values
				* can contain quotation marks, such as an apostrophe in the content.
				*/
				'content=(["\']??)(.*)\1' .

				/*
				* Allows for additional attributes after the content attribute.
				* Searches for anything other than > symbol.
				*/
				'[^>]*' .

				/*
				* \/?> searches for the closing > symbol, which can be in either /> or > format.
				* # ends the pattern.
				*/
				'\/?>#' .

				/*
				* These are the options:
				* - i : case insensitive
				* - s : allows newline characters for the . match (needed for multiline elements)
				* - U means non-greedy matching
				*/
				'isU';

		preg_match_all( $pattern, $html, $elements );

		return $elements;
	}

	/**
	 * Gets the metadata from a target meta element.
	 *
	 * @since 5.9.0
	 *
	 * @param array  $meta_elements {
	 *     A multi-dimensional indexed array on success, else empty array.
	 *
	 *     @type string[] $0 Meta elements with a content attribute.
	 *     @type string[] $1 Content attribute's opening quotation mark.
	 *     @type string[] $2 Content attribute's value for each meta element.
	 * }
	 * @param string $attr       Attribute that identifies the element with the target metadata.
	 * @param string $attr_value The attribute's value that identifies the element with the target metadata.
	 * @return string The metadata on success. Empty string if not found.
	 */
	private function get_metadata_from_meta_element( $meta_elements, $attr, $attr_value ) {
		// Bail out if there are no meta elements.
		if ( empty( $meta_elements[0] ) ) {
			return '';
		}

		$metadata = '';
		$pattern  = '#' .
				/*
				 * Target this attribute and value to find the metadata element.
				 *
				 * Allows for (a) no, single, double quotes and (b) whitespace in the value.
				 *
				 * Why capture the opening quotation mark, i.e. (["\']), and then backreference,
				 * i.e \1, for the closing quotation mark?
				 * To ensure the closing quotation mark matches the opening one. Why? Attribute values
				 * can contain quotation marks, such as an apostrophe in the content.
				 */
				$attr . '=([\"\']??)\s*' . $attr_value . '\s*\1' .

				/*
				 * These are the options:
				 * - i : case insensitive
				 * - s : allows newline characters for the . match (needed for multiline elements)
				 * - U means non-greedy matching
				 */
				'#isU';

		// Find the metadata element.
		foreach ( $meta_elements[0] as $index => $element ) {
			preg_match( $pattern, $element, $match );

			// This is not the metadata element. Skip it.
			if ( empty( $match ) ) {
				continue;
			}

			/*
			 * Found the metadata element.
			 * Get the metadata from its matching content array.
			 */
			if ( isset( $meta_elements[2][ $index ] ) && is_string( $meta_elements[2][ $index ] ) ) {
				$metadata = trim( $meta_elements[2][ $index ] );
			}

			break;
		}

		return $metadata;
	}
}