What's the correct way to create BNF for nested expressions, for custom language? Follow

Hi, I'm trying to write BNF file for my custom language plugin. I'm getting confused with the rules for nested expressions. My custom language contains both binary operator expressions and array reference expressions. So I wrote the BNF file like this:

{
  extends(".*_expr")=expr
  tokens=[
    id="regexp:[a-zA-Z_][a-zA-Z0-9_]*"
    number="regexp:[0-9]+"
  ]
}

expr ::=  binary_expr| array_ref_expr | const_expr

const_expr ::= number
binary_expr ::= expr '+' expr
array_ref_expr ::= id '[' expr ']'

But when I tried to evaluate expressions like 'a[1+1]' , I got an error:

']' expected, got '+'

What would I need to do?

Many thanks

Hao