WP_Block_Parser::next_token

	function next_token() {
		$matches = null;

		/*
		 * aye the magic
		 * we're using a single RegExp to tokenize the block comment delimiters
		 * we're also using a trick here because the only difference between a
		 * block opener and a block closer is the leading `/` before `wp:` (and
		 * a closer has no attributes). we can trap them both and process the
		 * match back in PHP to see which one it was.
		 */
		$has_match = preg_match(
			'/<!--\s+(?P<closer>\/)?wp:(?P<namespace>[a-z][a-z0-9_-]*\/)?(?P<name>[a-z][a-z0-9_-]*)\s+(?P<attrs>{(?:(?:[^}]+|}+(?=})|(?!}\s+\/?-->).)*+)?}\s+)?(?P<void>\/)?-->/s',
			$this->document,
			$matches,
			PREG_OFFSET_CAPTURE,
			$this->offset
		);

		// if we get here we probably have catastrophic backtracking or out-of-memory in the PCRE.
		if ( false === $has_match ) {
			return array( 'no-more-tokens', null, null, null, null );
		}

		// we have no more tokens.
		if ( 0 === $has_match ) {
			return array( 'no-more-tokens', null, null, null, null );
		}

		list( $match, $started_at ) = $matches[0];

		$length    = strlen( $match );
		$is_closer = isset( $matches['closer'] ) && -1 !== $matches['closer'][1];
		$is_void   = isset( $matches['void'] ) && -1 !== $matches['void'][1];
		$namespace = $matches['namespace'];
		$namespace = ( isset( $namespace ) && -1 !== $namespace[1] ) ? $namespace[0] : 'core/';
		$name      = $namespace . $matches['name'][0];
		$has_attrs = isset( $matches['attrs'] ) && -1 !== $matches['attrs'][1];

		/*
		 * Fun fact! It's not trivial in PHP to create "an empty associative array" since all arrays
		 * are associative arrays. If we use `array()` we get a JSON `[]`
		 */
		$attrs = $has_attrs
			? json_decode( $matches['attrs'][0], /* as-associative */ true )
			: $this->empty_attrs;

		/*
		 * This state isn't allowed
		 * This is an error
		 */
		if ( $is_closer && ( $is_void || $has_attrs ) ) {
			// we can ignore them since they don't hurt anything.
		}

		if ( $is_void ) {
			return array( 'void-block', $name, $attrs, $started_at, $length );
		}

		if ( $is_closer ) {
			return array( 'block-closer', $name, null, $started_at, $length );
		}

		return array( 'block-opener', $name, $attrs, $started_at, $length );
	}