I would suggest using ^
and $
to make sure nothing else is there:
[[ "$MAC_ADDRESS" =~ ^([a-fA-F0-9]{2}:){5}[a-fA-F0-9]{2}$ ]] && echo "valid" || echo "invalid"
EDIT: For using regex on BASH ver 3.1
you need to quote the regex, so following should work:
[[ "$MAC_ADDRESS" =~ "^([a-fA-F0-9]{2}:){5}[a-fA-F0-9]{2}$" ]] && echo "valid" || echo "invalid"