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Questions tagged [context-free-grammar]

In formal language theory, a context-free grammar (CFG) is a grammar subject to a special constraint: that the left-hand side (LHS) consist of a single non-terminal symbol. CFGs are capable of representing the set of context-free languages (CFLs).

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Drop-in, portable parsing

I see umpteen posts a day about "how to do X with regexen". And the best response to most of them seems like it would honestly be, "Why are you trying to drive a screw with a hammer?" But regexen are everywhere, and the syntax is mostly portable,…
Jeremy W. Sherman
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Is "regex" in modern programming languages really "context sensitive grammar"?

Over the years, "regex" pattern matching has been getting more and more powerful to the point where I wonder: is it really just context-sensitive-grammar matching? Is it a variation/extension of context-free-grammar matching? Where is it right now…
notnot
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Any tools can randomly generate the source code according to a language grammar?

A C program source code can be parsed according to the C grammar(described in CFG) and eventually turned into many ASTs. I am considering if such tool exists: it can do the reverse thing by firstly randomly generating many ASTs, which include tokens…
W.Sun
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NLTK Context Free Grammar Genaration

I'm working on a non-English parser with Unicode characters. For that, I decided to use NLTK. But it requires a predefined context-free grammar as below: S -> NP VP VP -> V NP | V NP PP PP -> P NP V -> "saw" | "ate" | "walked" NP ->…
ChamingaD
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Identity expression, factor, and term?

I am learning context-free grammar, and I don't understand how to identity expression, factor and term in a programming language like C or C++. Suppose we have an assignment statement, id := E, where E is any arithmetic expression. What is a term?…
CppLearner
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Parsing a context-free grammar in Python

What tools are available in Python to assist in parsing a context-free grammar? Of course it is possible to roll my own, but I am looking for a generic tool that can generate a parser for a given CFG.
Yuval Adam
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self-taught compiler courses / good introductory compiler books?

Does anyone know of online course / university lectures that comprise a typical compiler course? I've had theory of computing but unfortunately my school didn't offer a course in compiler construction. I know there are lectures out there; I was…
larryq
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If we know a CFG only generates regular language, can we get the corresponding regular expression?

As we know, given a regular grammar, we have algorithm to get its regular expression. But if the given grammar is context-free grammar (but it only generates regular language), like S->aAb A->bB B->cB|d Is there any existing algorithm that…
JackWM
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Using Parsec to parse regular expressions

I'm trying to learn Parsec by implementing a small regular expression parser. In BNF, my grammar looks something like: EXP : EXP * | LIT EXP | LIT I've tried to implement this in Haskell as: expr = try star <|> try litE <|>…
Xodarap
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Genetic algorithm grammar induction program/code?

Does anyone know of a program that uses a GA to perform grammar induction/inference, I've read tonnes of research papers and articles on this stuff like Lankhorst and De Pauw but I can't find any implementations or programmes that use this technique…
Matt Robinson
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Context free grammar for non-palindrome

I need a CFG which will generate strings other than palindromes. The solution has been provided and is as below.(Introduction to theory of computation - Sipser) R -> XRX | S S -> aTb | bTa T -> XTX | X | X -> a | b I get the general idea…
Abhijith Madhav
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Pumping Lemma with Context Free Languages

I have the language {a^i b^j c^k | i,j,k>=0 & i>j & j>k} I began by assuming some m is picked for me, such that a string z = a^m b^(m-1) c^(m-2) Then the string is split up into (z =) uvwxy so that vx are not empty and #(vwx)<=m Then when I get…
Alex
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How does one implement wildcards, character classes, negated character classes, etc. in a model for a regular grammar?

TL;DR: How does one computationally model a grammar's productions such that an indefinite number of products exist for the same left hand side? I'm working on a project regarding formal language theory and am trying to write a class for building…
Tyler Crompton
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What is the difference between Lexical grammar and Syntactic grammar?

I am reading The Java Language Specification 8. I am trying to understand Chapter 2. Grammars. Here's what I have already learned: Semantics: Semantics is the study of meaning. Meaning: Meaning, in semantics, is defined as being Extension: The…
Rounak
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First and follow of the non-terminals in two grammars

Given the following grammar: S -> L=L s -> L L -> *L L -> id What are the first and follow for the non-terminals? If the grammar is changed into: S -> L=R S -> R L -> *R L -> id R -> L What will be the first and follow ?
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