SHELL Regex and directory

Is there any easy way to check, if regular expr isnt some directory? More specificly. When there is dir path on the third argument ($3) and the regular expr (regex) in the second one ($2) can I check (probably with if statement) if the regex do not "cover" whole root directory?

some like

if [ $2 == $3 ]; then
 printf "Error. Regex cannot cover directory %s" "$3"
 exit 1
fi

Thank you