问题
Can someone please explain in layman's terms:
what a context-free grammar is?
what Backus Naur Form is?
How to use this notation?
How to do string derivation?
How to describe language syntax?
回答1:
A context-free grammar (CFG) G is a quadruple (V, Σ, R, S) where
- V: a set of non-terminal symbols
- Σ: a set of terminals (V ∩ Σ = Ǿ)
- R: a set of rules (R: V → (V U Σ)*)
- S: a start symbol
Example of CFG:
- V = {q, f,}
- Σ = {0, 1}
- R = {q → 11q, q → 00f, f → 11f, f → ε}
- S=q
- (R= {q → 11q | 00f, f → 11f | ε })
As far as I understand, Backus Naur Form (BNF) is another way of representing the things that are shown in the Context-Free Grammar (CFG)
Example of BNF:
[number] ::= [digit] | [number] [digit]
[digit] ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
which can be read as something like: "a number is a digit, or any number followed by an extra digit" (which is a contorted but precise way of saying that a number consists of one or more digits) "a digit is any one of the characters 0, 1, 2, ... 9"
Difference:
Notation of these two representations are a bit different, i-e
--> equals ::=
| equals or
there must be some other differences but to be honest I don't know any other :)
String Derivation:
Let S be the start" symbol
- S --> A, the initial state
- A --> 0A
- A --> 1B
- A --> ?
- B --> 1B
- B --> ?
Example of String Derivation:
Does this grammar generate the string 000111?
yes, it does!
- S --> A
- A --> 0A
- 0A --> 00A
- 00A --> 000A
- 000A --> 0001B
- 0001B --> 00011B
- 00011B --> 000111
that's all from my side, I too am working on it and will surely share if I come to know anymore details about defining the language syntax.
cheers!
回答2:
A context-free grammar is a type of formal language.
Backus Naur form is a specification language for this type of grammar. It is used to describe language syntax.
You should read:
http://en.wikipedia.org/wiki/Formal_language_theory
http://en.wikipedia.org/wiki/Context-free_grammar
http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form
来源：https://stackoverflow.com/questions/6641017/what-are-context-free-grammars-and-backus-naur-form