Learning how to write parsers by implementing an expression evaluator. Uses recursive-descent parsing technique. Supports variables.

Grammar:

Statement :=
    | ID "=" Expression
    | Expression
    ;

Expression :=
    | Term ( ( "+" | "-" ) Term )*
    ;

Term := 
    | Primary ( ( "*" | "/" ) Primary )*;
    ;

Primary :=
    | Factor ( "^" Primary)*
    ;

Factor :=
    | NUMBER
    | ID
    | ( "-" | "+" ) Factor
    | "|" Expression "|"
    | "(" Expression ")"
    ;