Inverting this particular Regex

Question

Using Powershell, I have this string that needs parsing :

,,Server.domain.abc.com,I1234

I'm trying to return only the part after the last "," character (here : I1234)

I sucessfully found how to isolate it using :

[^,]+$

https://regexr.com/471q7

but how do i invert this match so I can REPLACE all that is NOT this part with nothing.

My goal is to be able to do :

$var -replace "XXXX"

and end up with only that last part.