Parsing percent expressions with antlr4 - parsing

Question

I'm trying to parse algebraic expressions with ANTLR4. One feature I tried to accomplish with my parser is the "intelligent" handling of percent expressions.
Edit: The goal is to make the calculation of discounts or tips in a restaurant easier. E.g. if you see an advert "30% off" you could enter "price - 30%" and get the correct result. Or in a restaurant you could enter the price of your meal plus 15% and get the sum you have to pay including a tip of 15%. But this interpretation should only occur if the expression looks like "expression1 (- or +) expression2". In all other cases the percent sign should be interpreted as usual. The Google Search box calculator behaves like that./Edit
100-30% should return 70
100-(20+10)% should also return 70
3+(100-(20+10)%) should return 73
but
5% should return 0.05
(5+5)% should return 0.10
My grammar looks like this:
expr:
e EOF
;
e:
'-'a=e
| '(' a=e ')'
| a=e op=(ADD|SUB) b=e '%'
| a=e op=(ADD|SUB) b=e
| a=e'%' //**PERCENTRULE**
| FLT
;
ADD : '+' ;
SUB : '-' ;
FLT: [0-9]+(('.'|',')[0-9]+)?;
NEWLINE:'\r'? '\n' ;
WS : [ \t\n]+ -> skip ;
For the expression 100-30% I would expect the this tree:
But I get this:
How can I get the correct tree (without deleting PERCENTRULE)?

Answer 1

I deleted my original grammar-based answer because I realized I had a very different idea of what kind of handling you were trying to accomplish. It sounds like you want anything in the form X op Y % to become X * (1 op (Y/100)) instead. Is that accurate?
One feature I tried to accomplish with my parser is the "intelligent" handling of percent expressions:
Are you sure your specification for this is solid enough to even begin coding? It looks quite confusing to me, especially since % is more like a units-designation.
For example, I would have expected 50-30% to be either one of these:
(50 - 0.3) = 49.3
(50 - 30) / 100 = 0.20
...but what you're asking for sounds stranger still: 50 * (1 - 0.3) = 35.
That opens up additional weirdness. Wouldn't both of these be true?
0+5% would become 0 * (1 + 0.05) = 0
5% would become 5 / 100 = 0.05
This is odd because adding zero usually doesn't change what the number means.
A more-restrictive version
OK, what about allowing percentage-based changes only if the user avoids ambiguity? One way would be to create new binary operators like A -% B or A +% B, but that's not quite human-centric, so how about:
expr: e EOF ;
e
: SUB e
| parenExpr
| percentOp
| binaryOp
| FLT
;
parenExpr
: LPAREN e RPAREN
;
percentOp
: (FLT|parenExpr) (ADD|SUB) (FLT|parenExpr) PCT
;
binaryOp
: e (ADD|SUB|MUL|DIV) e
;
PCT : '%';
LPAREN : '(';
RPAREN : ')';
ADD : '+' ;
SUB : '-' ;
MUL : '*' ;
DIV : '/' ;
FLT: [0-9]+(('.'|',')[0-9]+)?;
NEWLINE:'\r'? '\n' ;
WS : [ \t\n]+ -> skip ;
This would mean:
50-5-4% is treated as 100-(5-4%) to get 45.2.
5% is not valid (on its own)
5%+4 is not valid