batch file - Control Characters and String Manipulation -

using trick posted on ss64 jeb save backspace control character variable - strange results when 'overlapping' backspaces.

the example in code golfing challenge posted here: backhanded^h^h^h^h^h^hspaces (would appreciate tips code shorter).

ungolfed this:

@echo off setlocal enabledelayedexpansion  /f %%a in ('"prompt $h&echo on&for %%b in (1) rem"') set del=%%a  set s=%1 echo %s:^h=!del!% 

basically, uses jeb's trick save backspace variable, uses string manipulation replace instances of ^h in input string variable. works correctly following example, , similar inputs:

c:\>bsp.bat "testing^h^h^h test" "test test" 

however when 'overlapping' usage of ^h occurs, following confusing (to me) output:

c:\>bsp.bat "aaa^hb^h^h" "a"b 

my question - above example, why receiving output instead of expected "a"?

i don't understand why "expected output" should "a". see character-by-character output:

bsp.bat "aaa^hb^h^h" " "a "aa "aaa "aaa    <- backspace, cursor on last "a" "aab "aab    <- backspace, cursor on last "b" "aab    <- backspace, cursor on second "a" "a"b 

the bs character moves cursor 1 character back. "standard behaviour" of backspace key (also called "backspace echo") "backspace+space+backspace".

by way, method used bs character:

for /f %%a in ('echo prompt $h ^| cmd') set bs=%%a