"Bullet Proof" Validation in Batch - validation

Question

This is a fairly general question on user input validation, with particular focus on special characters. In this "simplest" case one character validation is sought.
The code is modified from a 5 year old thread but it is apparent that most of the special characters require extra treatment to what is proposed below:
Setlocal EnableDelayedExpansion
:Prompter
REM Eliminate poison chars on <CR>
SET "validateText="
SET /P "validateText=Type f to foo or b to bah. (f/b)"
call :Validate
if /i %validateText%==f (echo foo pause>nul goto :eof
) else (
if /i %validateText%==b (echo bah pause>nul goto :eof
) else (goto Prompter))
:Validate
SET Input=%validateText:~0,1%
For %%G In (^= ^| ^& ^< ^> ^^ + ^( ^) \ / . # # $ { } [ ] ' : ` ^%% ^") Do (
REM Test Script fails for = | & < > ^ ) ; " and space
REM Loops through the set, but the if clauses are never evaluated
If %Input%==%%G (#echo isequal pause>nul exit /b) else (echo isnotequal pause>nul exit /b))
REM quotes on the test clause also fail
rem If "%validateText%==%%G" (echo equal pause >nul goto :eof) else (echo not equal pause >nul goto :eof))
REM The following are what is intended. All fail
rem If "%validateText%==%%G" (set Input=!Input:%%G=! goto BangDO) else (set Input=%validateText% goto :eof))
rem If %validateText%==%%G (set "Input=%Input:%%G=%" goto BangDO) else (set "Input=%validateText%" goto :eof))
:BangDO
#echo %Input%
pause
Set Input=!Input:%=!
REM Remove !
SetLocal DisableDelayedExpansion
Set "%Input:!=X"
Setlocal EnableDelayedExpansion
Set "validateText=%Input%"
#echo %validateText%
pause
The question is, that given there haven't been updates to the Command processor (at least since the quoted post) why it doesn't work. Either improve on this attempt at validation or provide own code.

Answer 1

The answer is almost staring you in the face :-) You enable delayed expansion, but then you fail to use it!
One beautiful feature of delayed expansion is you never have to worry about poison characters in the content.
One other thing, set /p will preserve any existing variable value if the user simply presses <Enter> without entering a value. That may or may not be what you want. I usually explicitly clear the variable value before using set /p
setlocal EnableDelayedExpansion
REM set /p does not clear existing value if user enters empty string
REM Remove REM below if you want to ignore any existing str value
REM set "str="
:prompter
set /p "str=Type f to foo or b to bah. (f/b) "
if /i !str!==f (
echo foo
) else if /i !str!==b (
echo bah
) else goto prompter
pause >nul

Answer 2

In your case only one replacement is necessary to be secure.
SET "validateText="
SET /P "validateText=Type f to foo or b to bah. (f/b)"
set "validateText=%validateText:"=""%"
if /i "%validateText%"=="f" (
echo foo pause>nul goto :eof
) else if /i "%validateText%"=="b" (
echo bah pause>nul goto :eof
) else (
goto Prompter
)
This works as all special characters are safe inside quotes.
And you only need to be sure that a quote inside your text can't cause problems, this can be achieved by doubling all quotes.