WP_REST_URL_Details_Controller::get_metadata_from_meta_element() WordPress Method

	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;
	}