Questions tagged [regular-expressions]
Questions about regular expressions, a formalism to describe regular languages.
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questions
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Regular expressions accepted by programming languages but are not regular [duplicate]
The below definition of Regular Language is given in Wikipedia.
In theoretical computer science and formal language theory, a regular language (also called a rational language) is a formal language ...
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1answer
36 views
a^ib^jc^k, i < j < k is a context-sensitive language, how can prove it as a context sensitive
I've been pondering this question for a long time, that $a^ib^jc^k, i < j < k$ is a context-sensitive language, how we can prove it to be context sensitive or
which grammar can generate such a ...
0
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1answer
35 views
How to apply Arden's theory to determine a regular expression
If $P=ab$ and $Q=a^*$,
how do I use Arden's theorem to determine the regular expression $R$.
I'm not sure if I am supposed to just substitute the values of $P$ and $Q$ in the equation $R= Q + RP$. ...
0
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2answers
40 views
Can a regular expression be any string from the language described by it? [closed]
Is it possible to have a regular expression from a language (that has strings of infinite length) which it describes ?
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0answers
29 views
Constant single match regex
I am looking for the name (definition?) of X in:
A regular expression is X iff it has exactly one possible match.
Examples: <empty regex>, abc, ...
2
votes
1answer
60 views
Extracting regex submatch boundaries without backtracking
I'm attempting to develop from scratch a simple regex engine. I would like for my engine to have the ability to report, "The regex matched on a substring of this line starting at index and ...
1
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1answer
42 views
Regular expressions
I am new to theoretical cs and am working through it slowly. I have completed a simple question and was wondering if my answer is correct or not. Thank you kindly for any feedback.
The question is:
...
1
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2answers
61 views
Does a regular expression exist for any number that contains no more than two 5s and no 6 twice in a row?
For example, a valid number would be 6165156 and an invalid number would be 1566515.
I have tried many times to construct a finite state machine for this with no success, which leads me to believe the ...
0
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1answer
91 views
CFG-Infinite recursion
As you see, the string production process never ends.
Can someone explain me if this language is regular or not ?
$ S \to Ī Ī S $
$ A \to S $
$ B \to a B b $
2
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1answer
37 views
Pumping Lemma,regular languages
Lets say that we have the language L = { $a^n$$b^m$$c^{m+n}$ $|$ $m$,$n$ $>=0$ }
What is the way that i should follow to prove
that the language is not regular?
Assume that the language is ...
0
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1answer
32 views
validation of a pumping lemma proof for regular languages
I have the following regular expression:
Of course I could think of a word like $w=a^{m+2}b^{m+2}c^{2m+3}$ and continue with the proof BUT
I was just wondering, because $L$ is made up of a union of ...
0
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1answer
21 views
What is Pumping length for Union of Regular languages?
This is an exam question.
For E = {a,b}. let us consider the regular language
$L= \{x|x = a^{2+3k} or x=b^{10+12k}, k >= 0\}$
Which one of the following can be a pumping length (the constant ...
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0answers
25 views
I can't seem to convert a NFA to a regex. What am I doing wrong? [duplicate]
I have the following diagram :
I try to remove state 3:
Now I do this:
What do I do now to get my regular expression? I'm stuck.
4
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1answer
238 views
What is the computational complexity of “real-life” regular expressions?
Regular expressions in the sense as equivalent to regular (Chomsky type 3) languages know concatenation xy, alternation (x|y), ...
0
votes
1answer
26 views
Convert the given NFA to DFA
I am trying to find an DFA for the regular language given by the expression $L\left( aa^{\ast }\left( a+b\right) \right)$.
First simplifying $L\left( aa^{\ast }\left( a+b\right) \right)$ we get
$L\...
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1answer
29 views
Turing Machine construction of M=wwRw form
Construct a Turing machine for
M = {wvw| v, w ā {a, b}*, reversal(v) = w}.
I tried to imagine that I will have to divide the string into 3 equal parts and check if ...
1
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1answer
25 views
Is regex with backreference still able to be implemented by finite automata?
I recently learned that regex support backreference which enables referring to a matched group in the pattern itself. E.g. a regex (.)\1{2,} matches two or more ...
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1answer
20 views
How can I prove that the accepted language of a given DFA or NFA or REGEX is equivalent to a given language
I found this How do I verify that a DFA is equivalent to a NFA? but as it states it is not really a good question more of how can I check myself during an exam. Because as you might know to do this by ...
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1answer
37 views
Construct regular expression for given language
How to construct regular expression for language L={a,b,c} which contain all words starts with bab and ends with babc?
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0answers
49 views
Does the order of ripping off the states matter?
Do we get same regular expressions at the end regardless of the order that we take the states step by step? Can somebody prove by exemplification?
3
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1answer
113 views
Why is this language a regular language?
Came across this in a book, and I'm wondering why the following language is regular?
$$ L = \{a^nww^R: n \geqslant 0, w \in \{a,b\}^3\}$$
Is it correct to say that $$ \{a^n : n \geqslant 0\} $$ is a ...
0
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2answers
43 views
Regular expression for a palindrome of finite length?
I have a language $$ L = \{ww^R, w \in \{ab\}^5\}$$
I know this is a regular language because it is finite (since w can only be of length 5). I want to prove it's a regular language, so I'd create a ...
1
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1answer
37 views
How to create a regular expression for this language?
I have a language:
$$ \{a^jb^k \mid j \neq k \text{ and } j \equiv k \pmod 3 \}$$
I want to prove that this language is regular. My first thought was to create a regular expression that accounts for ...
1
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1answer
57 views
RE Construction: Strings of even length in alphabet {a,b,c} that contain exactly one a
i am reviewing older exams from my computational theories lesson in university and i am struggling to figure this one out:
Construct a regular expression for the following language: Strings of even ...
28
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11answers
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Unusual applications of regular expressions?
I am looking for some interesting applications of regular expressions.
Can you name any unusual, or unobvious, cases where regexes find their application?
1
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1answer
29 views
Construct regular expression that contains the substring
I need to write a regular expression for the following set of strings on Ī£ = {a, b, c} in which the number of bās is even, and contains abcba as a substring.
So far, the even number of b's can be ...
0
votes
0answers
22 views
Construct a DFA with reduced states equivalent to 10 + (0 + 11)0*1
I have found this question in a book. I could convert it into its equivalent DFA. But the problem is with the "reduced state" part. I think the answer given in the book is wrong. What would be the DFA ...
0
votes
2answers
37 views
convert RE to NFA
The Regular Expression i am having trouble with is the following a(b|bcc*)*c. My main concern is what do i do with the c*? I can lay the rest of the diagram correctly (i think) but that part is ...
2
votes
2answers
37 views
Proving non-regular language using the pumping lemma
I would like to verify that the following language is not regular.
I know that if the pumping lemma is not valid then the language is not regular. (but its not enough to prove that the language is in ...
1
vote
2answers
173 views
Over the alphabet {a,b,c,d}, how would i construct a NFA that only accepts strings that end with a letter that is already part of the string?
I've been trying to create a NFA that accepts strings that end with a letter that exists in the string. For example abcdb, cbdd, acac etc. while strings like abc aacd etc are not accepted since the ...
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votes
2answers
147 views
Regular expression without 00 as a substring
I want to create a regular expression for the language:
$L=\{w \in \{0,1\}\mid w \text{ does not contain 00 as a substring}\}.$
I've tried various things, but I can't seem to get the correct regular ...
2
votes
3answers
115 views
An approach to determine whether language is regular or not?
I have the following problem. I need to determine whether this language is regular or not:
$L:= \{ w \in \Sigma^*: \forall \alpha ā \Sigma, |w|_\alpha \text{ is even or divisible by 3}\}$
$|w|_\...
0
votes
1answer
32 views
Is this a valid DFA for (a|b)*abb?
I'm trying to come up with an alternate DFA for accepting (a|b)*abb than the one present in the dragon book. Here's the one I tried:
It seems valid to me but I'm ...
0
votes
0answers
12 views
How to write regular expression [duplicate]
The set of all strings over {0,1} such that every block of 4 consecutive symbol contain at least two 0's
Can you guys tell me what is the correct answer for this ?
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0answers
15 views
Regex vs line by line read efficiency - general concept question
Say, I have a file that is somewhere between 10-20 pages long. I need to extract 10-20 pieces of information from it that I can fairly easily extract using a series of regex-es or by running code that ...
0
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0answers
43 views
Regular grammars
I am fairly new to automata, regular expressions and regular grammar. I am struggling to understand how to define the regular expression (abc)* into one or several regular grammar expressions.
...
0
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0answers
52 views
McNaughton-Yamada Algotihm (1960) Regular Expression to DFA without passing through NFA
I have a homework problem here. It asks me to use the McNaughton-Yamada algorithm to first convert a regular expression to a DFA, and then minimize this using a partition argument. I can do the latter....
3
votes
1answer
115 views
Arden's Rule, DFA & NFA to regular expressions
I have been trying to figure out the Arden's Rule and the equational method to transform DFA's & NFA's to RE. I know what the rule state:
if x = s + xr
then x = sr*, with $s,r\in$ Regular ...
1
vote
2answers
92 views
Prove the following language is regular?
Assume $L_1$ is a regular language, and define:
$$L = \{wcv ā \{a, b, c\}^* \mid |w|_a + 2|v|_b ā” 3 \bmod 5, w, v ā L_1\}.$$
Show that $L$ is regular.
I first tried to prove by showing that the ...
1
vote
1answer
46 views
Are these REs equivalent?
I have to implement a compiler for a given language as part of an assignment.
The language is kept simple enough such that it can be fully expressed through REs.
My question lies with two of the REs ...
2
votes
2answers
62 views
regex of three repeating number
I need a regular expression, not one using coding syntax, to write an expression that has a vocabulary of 1,2,3 and every string in the language has at most one occurrence of the substring 222 but ...
0
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2answers
66 views
Generating regular expressions not containing abc
Let sigma = {a,b,c}. How do I generate a language L that does not containg abc?
Any guidance is appreciated!
1
vote
1answer
25 views
How to describe the languages of these regular expressions in plain English?
This is actually a question my lecturer gave us, so I know that it's kind of homework, but I tried to answer this and still had no luck.
The question is
Give English description of the languages ...
1
vote
1answer
67 views
Converting DFA to Regular Expression Using State Removal
I'm trying to convert the following NFA to a regular expression.
I've attached my work below and end up with the expression $aa^*bb^*$. As far as I can tell, this doesn't seem correct but I've been ...
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0answers
33 views
How to write a regular expression that excludes certain strings? [duplicate]
It's a homework problem.
Assume $\Sigma = \{a ,b\}$, I am asked to construct a regular expression that does not have both the substrings bba and abb.
My idea was: construct a regex that matches ...
1
vote
3answers
170 views
Large DFA to regex?
For an assignment for one of my courses, one of the questions is to provide a regular expression for the language:
"the set of strings such that the number of 0ās is divisible by six, and the number ...
0
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0answers
48 views
Prove the equivalence between regular expressions (cb*c+cb*b)* and (cc)*+(cc)*(cb)(b+c)*
I need to prove the equivalence of the following regular expressions:
(cb*c+cb*b)*
(cc)*+(cc)*(cb)(b+c)*
using the following equivalence rules:
ā¢ (1) (E + F) + G = E + (F + G)
ā¢ (2) E + F = F + E
...
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2answers
133 views
Why L' is not regular?
$$L'=\{ww|w\in L\}$$
I need to give an example of regular language L for which the concatenation of 2's $w$ gives $L'$ which is not regular.
How can I give such an example if according to closure ...
1
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2answers
102 views
Prove that the language $\{a^ib^i | i\geq 0\}$ is not regular? (Do we just consider $a^nb^n$, where $n$ is the pumping length?
I think to prove that $\{a^ib^i | i\geq 0\}$ is not regular, we just have to consider the string $a^nb^n$ (which is in the language) and apply the pumping lemma. But I'm not sure how to proceed using ...
0
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0answers
106 views
Strings that do not contain 11 as a substring
Today I had a couple of formal language lectures. The Instructor wrote a regular expression for the alphabet $\{0,1\}$ which does not include any string which includes "11" as a substring.
She wrote ...
