Angular, testing a regex from a Json object

I have the following situation

I have a json file with certain regular expressions that i use to validate some fields

I get the data from the file within a service and inject the service into my directive

So in my directive i have

var alphanumeric  = new RegExp(Rules.password.alphanumeric);

I'm using new RegExp based on some answers in SO.

Then in order to do the validations:

pass_alphanumeric.test(newVal) // This way doesn't work

But if i do it with the Regex it does work.

/[A-z]/.test(newVal) // This works!

However if i do a console.log(pass_alphanumeric) i get:

/[A-z]/

I'm kinda stuck in here and not sure how to do it,

Note I need to get it from a Json File since It's connected to the backend and the rules can be changed anytime by another person.