The flavor of EBNF used here is the same as that defined [http://karmin.ch/ebnf/index here], except a literal can't contain the character used for quoting, and an identifier can't contain whitespace or any of the characters = | ( ) { } [ ] . ; " '.

==A one-liner==

"a" {
    a = "a1" ( "a2" | "a3" ) { "a4" } [ "a5" ] "a6" ;
} "z"

Some valid inputs:

• a1a3a4a4a5a6
• a1 a2a6
• a1 a3 a4 a6

Some invalid inputs:

• a1 a4 a5 a6
• a1 a2 a4 a5 a5 a6
• a1 a2 a4 a5 a6 a7
• your ad here

==Arithmetic expressions==

{
    expr = term { plus term } .
    term = factor { times factor } .
    factor = number | '(' expr ')' .

    plus = "+" | "-" .
    times = "*" | "/" .

    number = digit { digit } .
    digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" .
}

Some valid inputs:

• 2
• 2*3 + 4/23 - 7
• (3 + 4) * 6-2+(4*(4))

Some invalid inputs:

• -2
• 3 +
• (4 + 3

==Some invalid EBNF==

a = "1";

{ a = "1" ;

{ hello world = "1"; }

{ foo = bar . }