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...).