The term is just a historical heritage. what does 256 bytes of internal memory (RAM) on a microprocessor mean? It's not possible. From automata theory its impossible to match a paliandrome of any lenght ( because that requires infinite amount of memory). What can be done with I/Q data which would otherwise be impossible? Could someone by any chance explain that RE character by character for a newbie? Actually, I modified the expression expression slightly, to read: I think what is happening is that while the outer pair of characters are anchored, the remaining inner ones are not. How does this Java regex detect palindromes? The proof relies on the facts that we can easily build a string that requires an arbitrarily large number of nodes, namely the string, a^x b a^x (eg., aba, aabaa, aaabaaa, aaaabaaaa, ....). In academic terms, a regular expression has specific boundaries (defines a DFA). Palindromes aren't just a word game. Do vector spaces without choice satisfy Cantor-Schroeder-Bernstein? Should I mention in my statement of purpose that I did not attend lectures in my last two years of undergrad? On my PHP 5.3 under Win XP Pro it fails on: aaaba The “regular expressions” used by programmers have very little in common with the original notion of regularity in the context of formal language theory. In reality, many regexp engines (Perl and it's relatives primarily) support backreferences which violate the academic definition (becoming NFA or even broader). At this level, the capturing group matched d. The backreference fails because the next character in the string is r. Backtracking again, the second alternative matches d. Now, \k'letter+0' matches the second a in the string. What is the rising action of faith love and dr lazaro? The regex engine must now try the second alternative inside the group "word". I do not want to hire someone who will try to do something the wrong way if he knows better one. The only failure I have found is the single-char string, but I can test for that separately. I would hope you'd give a little more explanation, to show that you really understand the limitations of regex. How to validate an email address using a regular expression? (? I will, however, generally call them “regexes” (or “regexen”, when I’m in an Anglo-Saxon mood). Palindromes aren't defined by a regular language. The backreference specifies +0 or the present level of recursion, which is 2. : radar), checking for letters only: \([a-z]\)\([a-z]\)[a-z]\2\1 So it seems we need a different regex for each possible word length. Thanks @PeterKrauss. Does Jerry Seinfeld have Parkinson's disease? PS: it is using a definition where "o" is not a palimbrome, "able-elba" hyphened format is not a palindrome, but "ableelba" is. Still, the actual regexp would be quite long, much longer than equivalent 4-line program (easy exercise for the reader: write a program that identifies palindromes). The fifth recursion fails because there are no characters left in the string for [a-z] to match. What is the best regular expression to check if a string is a valid URL? You can also do it without using recursion: Regarding the PCRE expression (from MizardX): Have you tested it? During the next two recursions, the group captures a and r at levels three and four. I totally disagree with Airsource Ltd when he says that "it's not possibles" is not the kind of answer the interviewer is looking for. Look at the right: what is the meaning of "regular expression" in the related questions? ^((.)(?1)*\2|.? As you can see, I edited this answer recently because the previous version was wrong (for three years, what a shame). In my opinion, hanging onto old belief that regular expressions in our field are anything but a programming concept is just counterproductive. How long will the footprints on the moon last? ?\3\2\1$, Why rest of the people have 13 but this is 19, This is a very interesting answer to a regex question. The whole point of .NET flavoured Regex is that they aren't. The material on this site can not be reproduced, distributed, transmitted, cached or otherwise used, except with prior written permission of Multiply. If you are on a personal connection, like at home, you can run an anti-virus scan on your device to make sure it is not infected with malware. The regex engine exits from the third recursion. When "o" and "able-elba" are palindrones, naming definition2. Performance & security by Cloudflare, Please complete the security check to access. Your IP: 149.56.14.230 PCRE does seem treat the recursion as an atomic group, while Perl allows backtracking within it. Here's a quote from Larry Wall, the creator of Perl itself: (…) generally having to do with what we call “regular expressions”, which are only marginally related to real regular expressions. How to change the file system of a partition in a RAID 1? ^((.)(?1)?\2|. Thats specifically why I pointed out that you would have to use named capture groups. It's not an appropriate Here's my answer to Regex Golf's 5th level (A man, a plan). @J.F. How do you use a variable in a regular expression? FOLLOWING a matches Zero or one of a, but ? However, for the intents and purposes, palindromes cannot be checked with a, @SteveMoser There is a good writeup of Ruby's regular expression engine (. Pagkakaiba ng pagsulat ng ulat at sulating pananaliksik? The regex engine must backtrack. In Ruby you can use \b(?'word'(? If we end up in an accepting state then we say that the machine "accepts" that word. It works for up to 7 characters with the browser's Regexp (I'm using Chrome 36.0.1985.143). Depending on how confident you are, I'd give this answer: I wouldn't do it with a regular • Number Palindromes. @John right, so in the context of the question Jose is right and hqt is wrong. They can't support it.". Now the regex engine enters the first recursion of the group "word". Only standard regexes are limited to regular langs. You are right. If there is ever a real-world application that requires identifying palindromes then it will almost certainly not include arbitrarily long ones, thus this answer would show that you can differentiate theoretical impossibilities from real-world applications. where a^x is a repeated x times. That’s called an anagram. Using recursive reference: modified based on the near last part http://perldoc.perl.org/perlretut.html, In ruby you can use named capture groups. It can be done in Perl now. This bit of wordplay is not the same thing as when you rearrange the letters of a word or phrase to spell another one. It works fine with any language, here an example adapted from the same source (link) as proof-of-concept, using PHP: The regular expression ^((\w)(?:(?1)|\w? If you are at an office or shared network, you can ask the network administrator to run a scan across the network looking for misconfigured or infected devices. and it gives a lot of answers in different languages, but no answer that uses regular expressions. A slight refinement of Airsource Ltd's method, in pseudocode: To subscribe to this RSS feed, copy and paste this URL into your RSS reader.

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