Regular expression to control date format

Hi,
I'm running a script sh, with a date formatted as "YYYYMMDD", as
argument (example : ./my_prog.sh 20111216
In my script, I just would like to test if the user specified an
argument respecting the folowing rules :
- 8 characters long
- constituted only by numbers

I tried to write this :
if [[$1 =~ /^[2][0][1-2][0-9][0-1][0-9][0-3][0-9]$/ ]]
then
..
fi

But it does not work;

Any suggestion ?

Thanks,
Christophe