StrRep.nsh

!define StrRep "!insertmacro StrRep"
!macro StrRep output string old new
    Push `${string}`
    Push `${old}`
    Push `${new}`
    !ifdef __UNINSTALL__
        Call un.StrRep
    !else
        Call StrRep
    !endif
    Pop ${output}
!macroend
 
!macro Func_StrRep un
    Function ${un}StrRep
        Exch $R2 ;new
        Exch 1
        Exch $R1 ;old
        Exch 2
        Exch $R0 ;string
        Push $R3
        Push $R4
        Push $R5
        Push $R6
        Push $R7
        Push $R8
        Push $R9
 
        StrCpy $R3 0
        StrLen $R4 $R1
        StrLen $R6 $R0
        StrLen $R9 $R2
        loop:
            StrCpy $R5 $R0 $R4 $R3
            StrCmp $R5 $R1 found
            StrCmp $R3 $R6 done
            IntOp $R3 $R3 + 1 ;move offset by 1 to check the next character
            Goto loop
        found:
            StrCpy $R5 $R0 $R3
            IntOp $R8 $R3 + $R4
            StrCpy $R7 $R0 "" $R8
            StrCpy $R0 $R5$R2$R7
            StrLen $R6 $R0
            IntOp $R3 $R3 + $R9 ;move offset by length of the replacement string
            Goto loop
        done:
 
        Pop $R9
        Pop $R8
        Pop $R7
        Pop $R6
        Pop $R5
        Pop $R4
        Pop $R3
        Push $R0
        Push $R1
        Pop $R0
        Pop $R1
        Pop $R0
        Pop $R2
        Exch $R1
    FunctionEnd
!macroend
!insertmacro Func_StrRep ""
!insertmacro Func_StrRep "un."