Implemented in the Changes in Tcl/Tk 8.5, as part of TIP #272[1].

Workaround substitute for 8.4:

 proc string'reverse str {
    set res {}
    set i [string length $str]
    while {$i > 0} {append res [string index $str [incr i -1]]}
    set res
 } ;# RS
 % string'reverse "hello, world"
 dlrow ,olleh

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Implementation notes

This command converts its argument to the "unicode" object type so that it doesn't have to use Tcl_UtfPrev() masses of times.