Removing non alphanumeric characters in a batch variable

Question

In batch, how would I remove all non alphanumeric (a-z,A-Z,0-9,_) characters from a variable?

I\'m pretty sure I need to use findstr and a regex.

Answer 1

EDITED - @jeb is right. This works but is really, really slow.

@echo off
    setlocal enableextensions enabledelayedexpansion
    set "_input=Th""i\s&& is not good _maybe_???"
    set "_output="
:loop
    if not defined _input goto endLoop
    set "_buf=!_input:~0,1!"
    set "_input=!_input:~1!"
    echo "!_buf!"|findstr /i /r /c:"[a-z 0-9_]" > nul && set "_output=!_output!!_buf!"
    goto loop
:endLoop
    echo !_output!
    endlocal

So, back to the drawing board. How to make it faster? lets try to do as less operations as we can and use as much long substring as we can. So, do it in two steps

1.- Remove all bad characters that can generate problems. To do it we will use the hability of for command to identify these chars as delimiters , and then join the rest of the sections of god characters of string

2.- Remove the rest of the bad characters, locating them in string using the valids charactes as delimiters to find substrings of bad characters, replacing then in string

So, we end with (sintax adapted to what has been answered here)

@echo off

    setlocal enableextensions enabledelayedexpansion

    rem Test empty string
    call :doClean "" output
    echo "%output%"

    rem Test mixed strings
    call :doClean "~~asd123#()%%%^"^!^"~~~^"""":^!!!!=asd^>^^^ %%a in ("!input!") do ( 
            set "output=!output!%%a%%b%%c%%d%%e%%f%%g%%h%%i"
            set "input=%%j" 
        )
        if not defined input goto outPurgeCritical
    )
    goto purgeCritical
)
:outPurgeCritical

rem Step 2 - remove any remaining special character
(
:purgeNormal
    for /L %%z in (1 1 10) do (
        set "pending="
        for /f "tokens=1,* delims=%map%" %%a in ("!output!") do (
            set "output=!output:%%a=!"
            set "pending=%%b"
        )
        if not defined pending goto outPurgeNormal
    )
    goto purgeNormal
)
:outPurgeNormal

    endlocal & set "%~2=%output%"
    goto :EOF

Maybe not the fastest, but at least a "decent" solution