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Regular Expression Problem

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Regular Expression Problem
ajaymatrix
11 Jul, 2007 - 10:12 PM
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Given a set of alphabets S= {A,B,C,D}

Write a Regular Expression for Determining all possible combinations where the letter B occurs twice..

eg :

ABCB : valid
BBCD : valid
ABBB : invalid
DCBA : invalid


I know the code if the 2 B's occur together which is rather simple...

Any one has some idea ...
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KevinADC
RE: Regular Expression Problem
11 Jul, 2007 - 10:57 PM
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This must be school/course work. Post the code you have written so far to try and solve this question.
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ajaymatrix
RE: Regular Expression Problem
12 Jul, 2007 - 06:36 AM
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/BB/ is for 2 Bs occuring together..

but if the Bs are occuring at some other position then how to determine...
I am not that very good at Regex..

It was not school/course work.. this was the question asked in a popular web company's aptitude test...
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KevinADC
RE: Regular Expression Problem
12 Jul, 2007 - 09:16 AM
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That seems to have nothing to do with the question:


Write a Regular Expression for Determining all possible combinations where the letter B occurs twice..

Your regular expression is just checking if there are two consecutive B's in the string, it's not "determining all possible combinations". I don't think a regexp can do that. But if the strings are already constructed and all you need to do is see if there are two B's anywhere:

CODE
my $foo = 'BCBD';
if (2 == ($foo =~ tr/B/B/)) {
   print "Valid\n"
}
else {
   print "Invalid\n";
}    




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ajaymatrix
RE: Regular Expression Problem
12 Jul, 2007 - 10:14 AM
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hey..
that even i know if we know the string beforehand...
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KevinADC
RE: Regular Expression Problem
12 Jul, 2007 - 11:01 AM
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first you have to figure out all the possible permutations of two B's within the character set: A,B,C,D, for example, assuming only the B can be repeated:

ABBB
ABBC
ABBD
ACBB
ADBB

and so on.

I don't think you would even use a regular expression for that so the original question makes little sense to me. Is this apptitude test online?

This post has been edited by KevinADC: 12 Jul, 2007 - 11:02 AM
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ajaymatrix
RE: Regular Expression Problem
12 Jul, 2007 - 12:39 PM
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no the test was not online...

maybe they didn't want the exact soln.. maybe they were looking for the approach...
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KevinADC
RE: Regular Expression Problem
12 Jul, 2007 - 01:25 PM
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well, that is the question you posted so I am not sure what further advice to give you. Post the actual question if you can, if not, I guess thats the end of this discussion?
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ajaymatrix
RE: Regular Expression Problem
12 Jul, 2007 - 08:32 PM
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that was a written test and the question posted above was what was asked..
Maybe instead of a complete solution they were expecting what was your approach...


so the question remained pretty ambiguous and I couldn't think of any solution..

That is why I thought it would be a better to have a discussion in this forum..

Any way thanks for your precious time Kevin...
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KevinADC
RE: Regular Expression Problem
13 Jul, 2007 - 09:50 AM
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I am not sure if you are being saracastic or not about my precious time. If you're not, you're welcome. If you are, well, sorry you feel that way, I was just trying to help you the best I know how.
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PennyBoki
RE: Regular Expression Problem
13 Jul, 2007 - 10:27 AM
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Hi, I don't belong to this forum i.e. I know nothing about Perl or Python, but I wonder if this is the regular expression that you want
CODE
/[ABCD]*BB[ABCD]*/

I apologize if I intrude but I have to try.
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Amadeus
RE: Regular Expression Problem
14 Jul, 2007 - 12:05 PM
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Kevin is correct - regular expressions are a matching/description tool - they cannot build all possible permutations of a string.
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