Antlr4 grammar left recursive error - parsing

Question

I am having quite a problem with antlr4 right now.
Whenever I try to feed antlr with this RPN grammar
grammar UPN;
//Parser
expression : plus | minus | mult | div | NUMBER;
plus : expression expression '+';
minus : expression expression '-';
mult : expression expression '*';
div : expression expression '/';
//Lexer
NUMBER : '-'? ('0'..'9')+;
antlr will throw an error because plus,minus,mult and div are mutually left recursive.
I dont know how to fix that.
(I know this occurs because with this grammar "expression" could be infinitely looped, I have had this problem before with another grammar, but i could fix that on my own)
My only solution would be to restrict the grammar in the following way
grammar UPN;
//Parser
expression : plus | minus | mult | div | NUMBER;
exp2 : plus2 | minus2 | mult2 | div2 | NUMBER;
plus : exp2 exp2'+';
minus : exp2 exp2'-';
mult: exp2 exp2'*';
div: exp2 exp2'/';
plus2 : NUMBER NUMBER '+';
minus2 : NUMBER NUMBER '-';
mult2: NUMBER NUMBER '*';
div2: NUMBER NUMBER '/';
//Lexer
NUMBER : '-'? ('0'..'9')+;
but this is not really what i want it to be, because now i could work at maximum with expressions like
2 3 + 5 4 - *
and the grammar would be more complex than it actually could be.
Hope you guys can help me

Answer 1

ANTLR4 only supports "direct" left recursive rules, not "indirect", as you have them.
Try something like this:
grammar RPN;
parse : expression EOF;
expression
: expression expression '+'
| expression expression '-'
| expression expression '*'
| expression expression '/'
| NUMBER
;
NUMBER : '-'? ('0'..'9')+;
SPACES : [ \t\r\n] -> skip;
Btw, 23+54-* is not a valid RPN expression: it must start with two numbers.