# Tagged Questions

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### Describe in English the pattern in the following regular expressions [migrated]

Describe (in English phrases) the languages associated with the following regular expression. it says to be as simple as possible ...
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### $L=${$a^nb^nc^n : n \geq 0$} CFG Recognizing

Suppose $L=${$a^nb^nc^n : n \geq 0$} and I. $h(L), h(a)=a, h(b)=bb, h(c)=b$ II. $L^R$ III. $L^*$ IV. $h(L), h(a)=a, h(b)=bb, h(c)=a$ Why just I is a CFG and other is not? anyone can help me to ...
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### Language of a grammar vs regular expression vs nfa

I read some note about Automaton Course. i see this note, that following all is the same. but i think the L(g) is not equal to NFA and regular expression. anyone could help me with defining the ...
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### Regular expression translation.

Given a set {1,2,...9} how can I construct a regular expression starts with a 3 has no 8's and has even number of 6's? Here's what I tried:  Define a new set no8 = {1,2,3,4,5,6,7,9} ...
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### Regular Expressions Help [duplicate]

I need a little help with Regular Expressions. The allowed operations are obviously + (union) , * (Kleene star) and concatenation. I have to write Regex for the following 2 examples. I have tried a ...
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### Formal Languages - Regular Expression

I've been battling the following two questions for more than a day today. Write a regular expression (comprised of {a, b}) that contain at least two b and do not contain abb. Write a regular ...
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### Draw a finite state machine which will accept the regular expression $(a^2)^* + (b^3)^*$

Draw a finite state machine which will accept the regular expression: $(a^2)^* + (b^3)^*$ In particular, I am confused by the $+$ sign, what does it exactly mean? Most literature I could find about ...
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### Minimal DFA for a given regular expression

How can I construct a minimal DFA from the following definition? $L=\{w \in \{a,b,c\}^*$: if the second-to-last letter from w is an $a$, then the number of $c$'s $\le 1\}$ I've already made a ...
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### Finite state machine

I am doing discrete math, and we are studying Finite State Machines. But i am a little confuse on how to do this. Here is a question, Write a regular expression for the language, and define a finite ...
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### Prove or disprove whether the following is a regular language

I'm given the regular language L, and w being an element of L. If we remove the w from the language L, will the resulting language be still regular? Well I thought to be true. Since initially is a ...
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### What is the difference between regex operations in math and regex in UNIX / Linux?

What is the difference between regular expression operations (union, concatenation, kleene star) and regular expression (implemented in UNIX and can be used together with the grep command)? Are there ...
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### Checking some Regular Expression problems

I'm given the alphabet $$\Sigma = {\{a,b}\}$$ I tried to write a regular expressions for presenting the following sets: All strings in $$\Sigma ^ *$$ with: a-) number of 2s divisible by 4 b-) ...
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### Question about Notation in a Regular Language

I am a little confused about the following notation: $L' = \{xy|x\in L \ , y\in L^R\}$. I think this expression is not equivalent with palindrome but I am not entirely sure. For example, I think the ...
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### Algorithm for a regular expression

This was a question for an exam that I received 0 points on, I'd like to get some input on what the correct answer should have been. Imagine that you are asked to write an algorithm P that takes a ...
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### Give a regular grammar for L

Give a regular grammar for L= {a^n b^n : n<=100} I would do something like this : S---> A | empty string A---> aB| empty String B---> Ab but How do we keep count of the number in the grammar? ...
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### Is this proof using the pumping lemma correct?

I have this proof and it goes like this: We have a language $L = \{\text{w element of } \{0,1\}^* \mid w = (00)^n1^m \text{ for } n > m \}$. Then, the following proof is given: There is a $p$ ...
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### What are the states of this NFA?

I have to realize an NFA that recognizes the language of strings on the alphabet {a, b} ending with: bb, ba, baa. I thought that there must be the following states: $q_0$: the string ends with bb. ...
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### Prove min(L) = all words in L that they don't have any prefix of themselves in L

We define the minimal words language of $L, \min(L)$, to be the language of all words in $L$ that don't have any prefix in $L$. Assume $L$ is regular language. I need to prove by building an ...
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### Is this NFA correct?

For the language L = $\{\Sigma^*. 0 .\Sigma^5 . 1. \Sigma^*\}$ The NFA must have 8 states. Also, what would be the upper bound on the number of states of a DFA recognizing L.
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### Is this NFA correct for the language {w | w ends with an a and no a occurs between any occurrences of b}?

Language: {w | w ends with an a and no a occurs between any occurrences of b} The NFA must have exactly 3 states.
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### Is this DFA correct for the language $\{w \mid w \in \{ab\}^*\}$?

Language: $\{w \mid w \in \{ab\}^*\}$
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### For the regular expression, (a* + b*) . (a.b)* , does the following automaton recognise the language it describes?

I constructed the automaton below using the assumption that the language described by the regular expression above only accepted the following strings: Empty, aabab, babab, aaaabab, bbbabab etc ...
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### What is the difference between the automatas for the regular expressions (a + b)* and (a* + b*)?

I know that the automaton for the regular expression (a + b)* will just have one state, where the initial state = the accepting state and there is one edge going into that state labelled a,b. Sorry, ...
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### A regular expression for the words that don't contain the sequence $ab$ over $\{a,b,c\}$

The following is an exercise in a book I am reading: Let $\Sigma=\{a,b,c\}$, define $L$ to be the language of all words over $\Sigma$ that do not contain $ab$ as a sub-word. Find a regular ...
What are the rules of regular expression arithmetics ? For example: Let $\Sigma=\{0,1\}$ $1. 1+01=(\epsilon+0)1$. $2. (\epsilon+00)^*=(00)^*$