Comments on: Text parsing, formal grammars and BNF introduction

By: preeti

preeti — Sat, 30 Jan 2010 17:16:02 +0000

hi i need a source coding,to check the given expression in numbers is valid or not by using c programming

By: sar

sar — Fri, 23 May 2008 17:52:30 +0000

Hi,

I’am a student , and as a project I have to write a grammar (LBNF one) -and it’s ok for it-; then, I have to use the BNFC, and that’s th e pb i.e. Idon’t know how to use it.

Please help me. and thanks a lot.

Best regards.

By: ethana2

ethana2 — Mon, 21 Jan 2008 08:45:27 +0000

Lojban would have been perfect if it had a proper phonetic base.
After I fork it and make the Best Language on Earth, we can avoid this nonsense

By: RubenV

RubenV — Sun, 20 Jan 2008 19:52:46 +0000

Nicolas: You’re cheating there by combining recursion and regular expressions . Pure regular expressions can’t do that. Recursion is actually what sets regular languages and context free grammars apart.

By: Nicolas

Nicolas — Sun, 20 Jan 2008 18:30:02 +0000

no one in particular: indeed... I translated it somewhat badly. So to be correct:

whole_number ::= digit | nzdigit digits
sign ::= "-" | "+" | ""
integer ::= sign whole_number

Ruben: out of my head, if input is 'foo(bar(baz)bat)', can't you do something like

parts = []
regex = '^([.^(]*)[\(([.^)]*)\)]*(.*)$'

def parse(s):
    parts = regex.parse(s)
    parts.append(parts[0])
    for i in range(1, len(parts)):
        parse(parts[i])

parse(s)

By: RubenV

RubenV — Sun, 20 Jan 2008 17:46:42 +0000

Nicolas: Try expressing a language which recognizes balanced brackets, which can be arbitrarily nested using a regular expression.

Fairly trivial in BNF, impossible to implement using traditional regular expressions.

By: no one in particular

no one in particular — Sun, 20 Jan 2008 17:46:14 +0000

For the record, note that you’ve described the whole numbers, not the integers, because integers include negative numbers.

(I’m not trying to be annoying, but I figured it was worth pointing out. Thanks for the interesting read!)

By: Mattias Bengtsson

Mattias Bengtsson — Sun, 20 Jan 2008 14:32:19 +0000

I’ve been using BNFC (BNF Converter) for some compiler construction courses at school. I think it is a really neat project. It can generate Java, Haskell and C++-parsers. I’m not sure if it can output Python parsers though.

By: Nicolas

Nicolas — Sun, 20 Jan 2008 12:41:03 +0000

Anonymous: correct! Fixed, thanks.

Fok: I kinda dislike the non-Pythoness of Yakk, and as a result the kinda hard to understand syntax of PLY. Thanks for the pointer though

Ruben: can’t any BNF definition be translated into a set of regular expressions and controller code?

By: RubenV

RubenV — Sun, 20 Jan 2008 11:50:27 +0000

Interesting, but I think the issue of ambiguous grammars is missing. These bring their own class of problems.

Also, it might be interesting to mention the fact that regular expressions simply cannot represent the same class of languages as grammars. This is the very reason one would like to use grammars, which are inherently harder to parse (you need a PDA whereas a regular expression can be processed using a finite state automaton). On the other hand, the inexpressivity of regular expressions is the reason why they are used that often, they can be implemented very efficiently. You see, there is no free lunch

I can highly recommend Michael Sipser’s book on this topic: Introduction to the Theory of Computation. It’s a bit tough, but highly comprehensive (and very well written). More here: http://www-math.mit.edu/~sipser/book.html