The reason why nobody should use batch files. Found in an old batch file.

toLower.bat

:toLower str -- converts uppercase character to lowercase
::           -- str [in,out] - valref of string variable to be converted
if not defined %~1 EXIT /b
for %%a in ("A=a" "B=b" "C=c" "D=d" "E=e" "F=f" "G=g" "H=h" "I=i"
            "J=j" "K=k" "L=l" "M=m" "N=n" "O=o" "P=p" "Q=q" "R=r"
            "S=s" "T=t" "U=u" "V=v" "W=w" "X=x" "Y=y" "Z=z" "Ä=ä"
            "Ö=ö" "Ü=ü") do (
    call set %~1=%%%~1:%%~a%%
    :: it's nice how this line manages to represent the horrors of syntax in under 30 characters.
)