# Tagged Questions

Regular expressions or Regex is a search pattern for strings defined by a sequence of characters.

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### regex patterns not working [closed]

i am using this regex ^[[ \w\.\,\-\$\€\&]* ({[\w]+\:[\w]+})* ({=[\w ]+\.[\w]+})*]*$ and i want to match paterns like this ...
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### Factorising Cyclic expression .

What are ways for factorising cyclic expressions? Note: I am not saying about specific one. Just ways of factorising cyclic expressions.
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### Induction to prove regular expression

Prove that is if S and T are any regular expressions over the one-letter alphabet, (for example: Σ = {a}), and if n is any natural, then the languages (ST)^n and (S^n)(T^n) are equal. I have to use ...
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### Finding LCM of an expression

Image link I am currently trying to figure out how to solve this problem. I know how they got the denominator but am unsure how they got 6x in the numerator? Can somebody explain this to me?
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### Give a regular expression that generates C.

Question: In certain programming languages, comments appear between delimiters such as /# and #/. Let C be the language of all valid delimited comment strings. A member of C must begin with /# and ...
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### Write each of the following languages from the alphabet is ∑ = {0, 1} as a regular expression:? [closed]

a) All even length strings that end with 0 b) All odd length strings with a 0 in the middle. c)Strings that are divisible to 3 when interpreted as a positive binary number(for ex.,0011,001001,1100,...
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### equivalent regular expr

Given a regular expression $r_1$, is $(r_1^*)^* = (r_1^*)$. I know that this is true but I do not know how to prove it. Would it be adequate to set $r_1$ to a regular expression such as $(a+b)$ for ...
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### Use regular algebra to prove that (a + ab)*a = a(a + ba)* [closed]

I don't see how it's possible to reverse ab to ba.
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### relation on Languages of one finite machine !?

I adopted this question from 2013 Final Entrance Exam on CS. We have Finite Machine $M$ and Languages $L_1$ to $L_4$ as depicted in following picture: The question is which of the $A$ to $D$ ...
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### If $A^{2x}=4$ what is ${A^{3x}-A^{-3x}\over A^x - A^{-x}}$
If $A^{2x}=4$ and $A > 0$, what is the numerical value of $${A^{3x}-A^{-3x}\over A^x - A^{-x}}$$ Could anyone find the solution and answer? Thanks
Originating from Sipser's book: Let $A$ be any language, define $A_{{1\over3}-{1\over3}}$ be the subset of strings of $A$ whose middle third is removed. The solution I came across makes the ...