Converting EBNF to BNF

Question

It's been a few years since my computer-language class and so I've forgotten the finer points of BNF's and EBNF's and I don't have a textbook next to me. Specifically, I've forgotten how to convert an EBNF into BNF.

From what little I remember, I know that one of the main points is to convert

{ term }

into

<term> | <many-terms>

But I don't remember the other rules. I've tried to look this up online but I can only find links to either homework questions, or a small comment about converting terms with curly braces. I can't find an exhaustive list of rules that define the translation.

Nambypamby

Answer 1

See this page.^🕗 It contains instructions for each production that needs to be converted:

From EBNF to BNF

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For building parsers (especially bottom-up) a BNF grammar is often better, than EBNF. But it's easy to convert an EBNF Grammar to BNF:

• Convert every repetition { E } to a fresh non-terminal X and add

  X = ε | X E.
  

• Convert every option [ E ] to a fresh non-terminal X and add

  X = ε | E.
  

  (We can convert X = A [ E ] B. to X = A E B | A B.)

• Convert every group ( E ) to a fresh non-terminal X and add

  X = E.
  

• We can even do away with alternatives by having several productions with the same non-terminal.

  X = E | E'. becomes X = E. X = E'.

Monte

Perfect! That's exactly what I need. Someone I never found that link! – Nambypamby

I'm getting a 404 on that link now... any idea if there is an updated version somewhere? – Germano

Alas, not off the top of my head. Hopefully the page will come back. – Monte

The page has moved to lampwww.epfl.ch/teaching/archive/compilation-ssc/2000/part4/… – Subway

Answer 2

Be warned: EBNF as it's listed in the ISO standard also includes exceptions to syntactic rules, which do not have a BNF equivalent. The conversion given by 500 - Internal Server Error only works for the portion of EBNF which overlaps with RBNF/ABNF.

Millisecond