control the user part of the to header with HMR

Hi!

 

I need to use HMR to validate the lenght of the user part of the req header, so the number of digits don't exceed 10.

 

When there are 4-digit prefix + 10 digits on the user part the call should pass.

 

When there are not 4-digit prefix + 10 digits on the user part, the call should be dropped.

 

I tought of this regex, but it only matches the prefix+the 10 digits:

 

^\d{4}\d{10}\b$

 

And I need to perform the action when happens exactly the opposite (no 10 digits).

 

is this a good idea?

 

!(^\d{4}\d{10}\b$)

 

any othe conditions I should consider (store the value to match against the expression first, for example)