Serial format sintax not working

Hello,
I have set the random number generator sintax to
MED-[0-9]{7}

I would expect it to print
MED-7randomnumbers

but when I create a test subscription from backed it just keeps printing 15 random letters and numbers
exyt8W4qBftTqAR

is this because I am creating these subscriptions form backend, and the sintax will work only for those created from frontend directly?

Thank you

I just tried creating a new item in "Plugin: randomgen"
Hikaserial > Plugins > Random generator > new

this works,

MED-[0-9]{size}
MED-45655
MED-12270
MED-88560
MED-13875
MED-54763

but I am not sure how to attach this new plugin item I created it to a certain plan, I am probably missing something here in how the component works... will have to study the documentation a bit more, in the meantime any suggestion is appreciated

Thanks

Yes, I know.
At first I tried using the sintax generator inside the plan settings, I set it like this
MED-[0-9]{size}
serial size: 5
see nimb.ws/59ZPxu

but when I create a subscription from backend it create just 5 random letters, like:
WkmPf

I have also tried to set it to something like
[0-9]{5}
or
[A-Z]{5}
it always generates 5 random letters when creating a subscription form backend

Thanks