Postman Regular Expressions on Mock Request

I am currently trying to design some test cases for a project I'm working on and I want to simulate that the Post request gets denied of some data isn't correct.

I have the following request body (JSON format):

{
    "key": "key",
    "name": "a name",
    "latitude": 123.345,
    "longitude": 543.321,
    "shortName": "SHORT",
    "isDepot": false,
    "isReliefPoint": false
}

And in the example for this request I was trying to match ANY key that fitted my validations, in this case with a maximum of 5 characters:

{
    "key": "\b\w{1,5}\b",
    "name": "a name",
    "latitude": 123.345,
    "longitude": 543.321,
    "shortName": "SHORT",
    "isDepot": false,
    "isReliefPoint": false
}

Anyone knows a way to achieve what I am currently trying?