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I am struggling with a homework problem. I have tried this problem for hours literally. I found a similar question here, but it is not exactly my problem.
The homework problem says 1. (20 points) Construct regular expressions for the following languages.
a) All strings of digits with at most one repeated digit.
The only way I see how this is possible would be to exhaustively somehow take care of every possible case. There are 10 different digits, so it's like A LOT of different cases. I think the max length string can be 11, because after 11, you have to have a second repeated digit. So the number of possible combinations is 10^11. I thought even about writing a DFA and just converting it to a regex, but even that seems like it's impossible.
Does anyone have any advice? We aren't allowed to use non-standard regex features, like groups, lookahead, etc. This is just a plain old regex kind of problem.
Response to comment:
It is not binary. I already asked the teacher.
"Commenters, “regular expression” has one well-defined meaning in computer science. Since this is homework, it’s almost certainly that which is meant (and even more so as it talks of “languages”), and not some specific library. There’s no ambiguity here, and no clarification needed." This is basically what we want. The standard regex stuff often used in theoretical CS classes. As far as what we learned in class, I go to USC if anyone is familiar with that and we only barely talked about this at all. We're onto a completely different topic now.
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I'm aware regular expressions, as a concept, represent a single Regular Language, which can be processed via a DFA/NFA with O(n) ~O(2^m) complexity, being n the size of the string and m the size of the regex. Most stack-overflow discussions about the subject quote this awesome article that proves it.
However, regex implemented in modern languages deal with more than regular languages. For instance, it's possible to recognize palindromes with regex, a context-free grammar problem that, when solved via a push-down automata(PDA), is known to have a O(n³) complexity.
I would like to know were exactly in the Extended Chomsky Hierarchy modern (perl or python re, for example) regex implementations fit, or, at least, their worst possible complexity.
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so recently I was reading a book that teach you how to program in C++ and I am at basic math stuff. I'm not a beginner in C++ but I have seen something strange that I can't find on the internet.
It is a math expression: 5*3(6' 4) and it has a single quote on it. I found didn't know what it was so I checked up on the internet for this and I found only that it is referred as prime. But I don't think it solve my problem and I don't know how correct that is.
Thanks in advance.
EDIT: I want to address all people who commented and answered (and those who will to do the same thing)in this post that I HAVE MADE A BIG MISTAKE. The real math expression was: 5*6(6*4) but my friend's book(which I got this expression) was not printed well and it looked like the one I have wrote in the past. I'm really really sorry about this....
C++14 and up allows single quote ' inside integer literals to allow for grouping digits. Thanks to this, 123'456'789 can be used as more readable equivalent to 123456789.
Depending on a country, it may be also used in mathematical (outside of C++) expressions, although Polish people would rather use 123 456 789 and UK people would use a comma AFAIK - 123,456,789. None of these is valid in C++ (or it would yield results far from expected), but it would be reasonable in mathematical text/formula.
Nevertheless, your example still wouldn't compile. There is no operator between 3 and opening bracket (.
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I'm performing some regular expression exercises in Python 2.7.3, on Windows 7. Some of these exercises have me looking for similar patterns on the same line. For example, I want to use regex to capture name1 and name2...
<XML tag><more tags>[name1]</XML tag><XML2 tag>[name2]<XML2 tag></more tags>
Would it be "cheating" or "missing the point" if I used any string parsing to capture name2? I feel like using regex the correct way alone should be able to capture both of those names, but string parsing is what I've always been familiar with.
An analogy would be like someone studying recursion in C++, but using a While loop. Recursion should NOT have any While loops (although of course it may be part of some other grand design).
Good question! Many beginners come into it believing they should be able do everything with one regex match. After all, people are always saying how powerful regexes are, and what you're trying to do is so simple...
But no, the regex is responsible for finding the next match, that's all. Retrieving the substring that it matched, or finding multiple matches, or performing substitutions, that's all external to the act of matching the regex. That's why languages provide methods like Python's findall() and sub(); to do the kind of "string parsing" operations you're talking about, so you don't have to.
It occurred to me a while back that the process of mastering regexes is one of learning everything you can't do with them, and why not. Understanding which parts of the regex matching operation are performed by the regex engine, and which parts are the responsibility of the enclosing language or tool, is a good start.
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I need to learn how to change a transliteration of a text to another writing system. Apparently the best way would somehow involve regular expressions and perl, probably from command line? I've been using regular expressions earlier in Notepad++ and TextWrangler, so I know some basics already. If there is some really good (and relatively easy and customizable) way to do this in Ruby or something else, I can start learning that as well. There is a constant need to transliterate linguistic sample texts in my field in Uralic linguistics, where many different variants of transliteration systems are used. So it is worth investing some time.
So the material I have now consists of lines with a sentence on each line. Some lines have other data like numbers, but those should stay as they are. I want to keep the punctuation marks as they are, this is just about converting one set of unicode letter characters to another. I searched the site but a lot was about converting from ascii to unicode and so on - this is not the problem here.
So the original text is like this (in broad Finno-Ugric Transcription):
mödis ivan velöććyny pećoraö ščötövödnej kurs vylö.
And I would need it in a form like this:
мӧдiс иван велӧччыны печораӧ щӧтӧвӧднэй курс вылӧ.
This continues for some thousand lines.
There is a clear correspondence between characters used, but it is sometimes complex and involves dealing first with some digraphs and consonant + vowel combinations, etc. As you see from the example, in some situations latin i corresponds to cyrillic и but in some positions can remain as i. Different texts have different solutions, so I would need to adjust the rules in each case. I understand I would need to run a long series of regular expressions in a very specific order to make it work. This order I will figure out myself, but I need to know into what kind of tool I have feed these rules in and how to do it.
I also have often situations where I would like to have the original sentence and transliterated one separated by a tab, so that the lines would have a form like this:
mödis ivan velöććyny pećoraö ščötövödnej kurs vylö. мӧдiс иван
велӧччыны печораӧ щӧтӧвӧдней курс вылӧ.
Of course there are many more questions, but after learning these basics I think I can move forward independently. Learning this would help me a lot. Thanks in advance!
Niko
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I am supposed to build a program for storing and handling huge integers. I know that there are many answers out there but I need ideas that I can implement easily, bearing in mind that I can use any of the basic concepts of C/C++.
How should I go about it?
This is the first time I am asking a question here so please correct me if I am wrong about anything.
Edit: Actually what I wanted to know was how should I go about storing a huge integer... Obviously an array is what comes to mind at first glance but are there any other methods out there at the basic level?
EDIT2: I came across a very nice solution to this problem a while ago, but was just a bit lazy to put it on here. We can use the concept of number systems to deal with huge numbers. We can declare an array that holds the co-efficient of powers of 256, thus obtaining a base 256 system. We can then use fundamental concepts like those of the various number systems to obtain our required results.
Matt McCutchen has a Big Integer Library
If you want to do this yourself his code would be a great starting point. As you can overload arithmetic operators in C++ it is not too difficult to make a new BigInteger class and make this handle any number of bits per integer.
There is also a stack overflow answer to this question: here
I consider this as a question about theory, as such I suggest to browse the internet using the right keywords for documents/articles or to take a sneak peek at libraries that are implementing this feature and are well tested, this projects also tend to offer a mailing list or a forum where developers can communicate, it can be a good place to start writing about this stuff.