sed regexp for replacing xxx in define('DB_PASSWORD', 'xxxx');?

sed regexp for replacing xxx in define('DB_PASSWORD', 'xxxx');?

I'm looking for a regexp to quickly replace the password inside the
following wp-config file, like so:
define('DB_PASSWORD', 'xxxx');
Now the config file has many defines, so it essential to only replace the
DB_PASSWORD parameter. I also don't know the old password, meaning i can't
do a replace 'xxxx' with 'yyyy'.
I'm on Linux with SED.... (need to replace many files) so something like:
cat wp-config.php | sed 's/[here the regexp]/new password/'
Thanks!