can't avoid another intervening search, you must save and restore the
match data around it, to prevent it from being overwritten.
+ Notice that all functions are allowed to overwrite the match data
+unless they're explicitly documented not to do so. A consequence is
+that functions that are run implictly in the background
+(@pxref{Timers}, and @ref{Idle Timers}) should likely save and restore
+the match data explicitly.
+
@menu
* Replacing Match:: Replacing a substring that was matched.
* Simple Match Data:: Accessing single items of match data,