Allow variable to start with a letter

[ryanheart]

Detect variables by first character based on a regex that can be overridden. Default could be [a-zA-Z$].

This provides better usage for end users and closely resembles algebra

[ircmaxell]

The simple support mechanism would be to implement a regex. The problem comes in that the tokenizer right now will greedily consume digits. So you could literally never have a variable that contained a digit (as it would be tokenized separately). This may be ok, as we could support alpha variables (without the leading $)...

The clean solution would be to rewrite the current system into a proper lexer rather than a dirty tokenizer. But that may be beyond the scope of the tool...

[ryanheart]

I think for now I'd be happy with the inclusion of only alpha characters. Maybe just throw an exception if a variable is added that does not conform.

[ryanheart]

I've been thinking about this and I think the fact digits would not be allowed in the equation is a good thing.

For instance if you have 2y4x this would actually be 2 * y * 4 * x at the moment the code would pick up 2,x,4,x and I think this is right and no operator should be used as multiplication.

[ruskid]

Its 2016 and thank you for the parser.
Unfortunately using numbers in variables was a must for me. ($q1 + $q2) * $q3

The regex I am using in tokenize function is:

((\$[a-zA-Z]+\d+)|(\d+\.?\d+|\+|-|\(|\)|\*|/)|\s+)

But this regex makes leading $ required, as commented above.