@ echo off
setlocal enabledelayedexpansion
set L=abcdefghijklmnopqrstuvwxyz
>b.txt (for /f %%i in (a.txt) do (
set a=%%i
set b==
for /l %%j in (0 1 25) do set n=0&call :A !L:~%%j,1!
echo !b!
))
goto :eof
:A
if "!a:~%n%,1!"=="" goto :eof
if "!a:~%n%,1!"=="%1" set b=%b%%1
set /a n+=1
goto A