## proc sstring {src arg rpc}
proc sstring {args} { set all 0;set idx 0; if {[lindex $args 0] eq "-all"} { set args [lrange $args 1 end]; set all 1 }; lassign $args src arg rpc; # go on as usual with your 3 arguments
set len [string length $src]
set len [incr len -1]
# set idx -1
while {$idx != -1} {
set idx [string first $src $arg]
if {$idx != -1} {
# puts $idx
set arg [string replace $arg $idx [expr $idx+$len] $rpc]
# puts "$arg $idx $src"
if {$all != 1} {set idx -1}
}}
return $arg
}

example:


set input "\[{(niet doe maar niet toch niet"
set repl "welletjes"
set srch "niet"
#puts $input
puts [sstring $srch $input $repl]