NCalc using symbols in custom function name

Question

Let's say that in my expression input I like to make functions stand out with a special prefix like $:

var expr = "Your name is $proper([firstName]) $upper([lastName])";

where I would write custom functions for $proper and $upper.

However, I get console errors "no viable alternative at character '$'". It works if I get rid of the symbol, or use an underscore.

Is there a way to use symbol characters in function names in NCalc? This also doesn't work when the function begins with a number (as in, if I wanted to shorthand the function name double as 2x).

(Sort of similar to NCalc evaluation error no viable alternative at input ',')