checking for hexadecimal vals only in a string in bash

[Chris Kottaridis <chriskot@xxxxxxxxxxxxx>]

In a bash shell script I want to see if a string has nothing but 
hexadecimal values in it.

So:

A098FE or af098fe

should be true

hello

should not.

How do  I check for that ?

I've been playing with

if [[ $val =~ '/^[A-Fa-f0-9]+$/' ]] ; then
   echo is hex
else
   echo is not hex
fi

I've tried various incantations of above  and also tried using grep but 
can't seem to get it just right.

Any help appreciated.

Thanks
Chris Kottaridis
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