Read the information regarding [string] for the real details of this subcommand.

"string map ..." was introduced as one of the
[Changes in Tcl/Tk 8.1.1].

[[ [Regular expressions] are the weapon of choice for some
people, to the extent that they fail to notice superior RE-free
solutions.  "string map ..." is worth learning, in the sense
that it solves several important problems that are clumsy with
REs.]]

See [Counting characters in a string] on how [string map] is the best performer (again to [RS]'s surprise...) - because it gets byte-compiled since 8.4.