So, that's not a regex you want, but a wildcard. In that case, you could do something like:
for file in $HOME/folderName/*.xml; do
if [ ! -f $file ] then assert=0; fi
done
echo $assert;
But of course, beacuse you are going through the files, the -f is implied. This would still be useful if checking for things like whether it's a folder, readable, etc...