How do I get parameter inside value from its string representation in C#?

[sureshmss]

var str = "?$Expand=User($filter=substring(Name,1) eq 'First'&$Select=Name),Role($filter=contains(Name, 'admin' )), Group";

My input is User and need to get inside string like "$filter=substring(Name,1) eq 'First'&$Select=Name"

If pass input is Group it will return empty

How to achieve this functionality?