How to make a simple calculator syntax highlighting Follow

I'm making a custom language support plugin according to this tutorial and I'm stuck with a few .bnf concepts. Let's say I want to parse a simple calculator language that supports +,-,*,/,unary -, and parentheses. Here's what I currently have:

Flex:

package com.intellij.circom;

import com.intellij.lexer.FlexLexer;
import com.intellij.psi.tree.IElementType;
import com.intellij.circom.psi.CircomTypes;
import com.intellij.psi.TokenType;

%%

%class CircomLexer
%implements FlexLexer
%unicode
%function advance
%type IElementType
%eof{  return;
%eof}

WHITESPACE = [ \n\r\t]+
NUMBER = [0-9]+

%%

{WHITESPACE}    { return TokenType.WHITE_SPACE; }
{NUMBER}        { return CircomTypes.NUMBER; }

Bnf:

{
  parserClass="com.intellij.circom.parser.CircomParser"

  extends="com.intellij.extapi.psi.ASTWrapperPsiElement"

  psiClassPrefix="Circom"
  psiImplClassSuffix="Impl"
  psiPackage="com.intellij.circom.psi"
  psiImplPackage="com.intellij.circom.psi.impl"

  elementTypeHolderClass="com.intellij.circom.psi.CircomTypes"
  elementTypeClass="com.intellij.circom.psi.CircomElementType"
  tokenTypeClass="com.intellij.circom.psi.CircomTokenType"
}

expr ::=
   expr ('+' | '-') expr
  | expr ('*' | '/') expr
  | '-' expr
  | '(' expr ')'
  | literal;
literal ::= NUMBER;

First it complains that expr is recursive. How do I rewrite it to not be recursive? Second, when I try to compile and run it, it freezes idea test instance when trying to parse this syntax, looks like an endless loop.