How to split a number from a regex expression in c#?

I suggest extracting Matches instead of Split:

string exp = "$(2.1)+$(3.2)-tan($(23.2)) * 0.5";

var doubleArray = Regex
  .Matches(exp, @"\$\((?<item>[0-9.]+)\)")
  .OfType<Match>()
  .Select(match => match.Groups["item"].Value)
  .ToList();

Console.WriteLine(string.Join("; ", doubleArray));

Outcome:

2.1; 3.2; 23.2