I'm playing around with creating a parser in PHP for my own flavor of BNF, to match strings against grammar in this BNF variant. It's still a work in progress and subject to change (I may even end up switching to support ABNF or EBNF, in stead of my own variant), but it's coming along quite nicely.
Now, forgive me if I mix up nomenclature here, but I believe what I have created so far could be called a LL(1) parser (or perhaps it would be better to call it a lexer), in that it evaluates the source one character at a time, without backtracking.
In my BNF variant a production rule is terminated by a new line. But one thing, amongst other things, that a grammar defined in my BNF variant does not allow yet, is to have multi-line production rules. It surely isn't a necessity, but it would be a nice-to-have, for improved readability of the grammar rules.
However, I'm struggling to find a way to parse such a rule, without backtracking. I guess I could decide to have, for instance, a semicolon be the terminating character for a production rule. But just out of curiosity: should it, in principle, be possible to disambiguate newlines as either being part of a rule, or the end of a rule, without backtracking? Or is that simply not possible?
For your consideration, here is a small (non-exhaustive) example grammar in my BNF variant:
<DIGIT> ::= "0"-"9" <ALPHAHI> ::= "A"-"Z" <ALPHALO> ::= "a"-"z" <ALPHA> ::= <ALPHAHI> | <ALPHALO> <ALPHANUM> ::= <ALPHA> | <DIGIT>