It would help to know a little more about your data, but I did come up with something that may work for you.

I've never liked lookahead or lookbehind regex. I read that it should work in the RegEx action, but it definitely doesn't work JavaScript. 

Maybe try this:

>^{"A-z":,]d{0,4}‍

Here's the RegexTester link to show how it works: Parse JSON - Regex Tester/Debugger 

As long as the values you're trying to return are always numbers, this should work. If they can be longer than four digits, you can increase the {0,4} to whatever you need.

You could also use the patterns below with the RegEx action.

(?<=FacilityId"":)[^,]+‍‍

(?<=PatientId"":)"^,]+‍‍

You would need one Regular Expression action for each pattern above.  Each result would need to be stored in a collection variable, then retrieved using a Collection Operation action and stored in a workflow variable.

Just to answer my own question here:

(?<="FacilityId":)(?<FacilityId>[dw-]+)(?=,)

Yea, I found out the collection bit the other day. A little odd that it won't let you return the item to a single variable.