Not able to exclude an application from shadow using its class name

[fedxyz]

Hi,

I have an application running and I want to exclude it from having shadow.
Using xprop WM_CLASS on it I get:

WM_CLASS(STRING) = "supremo.exe", "supremo.exe"

So on shadow-exclude I tried to use:
"class_g = 'supremo.exe'" but this doesn't work

but this "class_g ~= '^supremo.*exe$'" works.

Why the first one doesn't work ? I suspect I have to escape the point, I tried \. but I get an error.

Thanks for the help.