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  • Using lookahead assertions in regular expressions

    - by Greg Jackson
    I use regular expressions on a daily basis, as my daily work is 90% in Perl (legacy codebase, but that's a different issue). Despite this, I still find lookahead and lookbehind to be terribly confusing and often unreadable. Right now, if I were to get a code review with a lookahead or lookbehind, I would immediately send it back to see if the problem can be solved by using multiple regular expressions or a different approach. The following are the main reasons I tend not to like them: They can be terribly unreadable. Lookahead assertions, for example, start from the beginning of the string no matter where they are placed. That, among other things, can cause some very "interesting" and non-obvious behaviors. It used to be the case that many languages didn't support lookahead/lookbehind (or supported them as "experimental features"). This isn't the case quite as much, but there's still always the question as to how well it's supported. Quite frankly, they feel like a dirty hack. Regexps often already are, but they can also be quite elegant, and have gained widespread acceptance. I've gotten by without any need for them at all... sometimes I think that they're extraneous. Now, I'll freely admit that especially the last two reasons aren't really good ones, but I felt that I should enumerate what goes through my mind when I see one. I'm more than willing to change my mind about them, but I feel that they violate some of my core tenets of programming, including: Code should be as readable as possible without sacrificing functionality -- this may include doing something in a less efficient, but clearer was as long as the difference is negligible or unimportant to the application as a whole. Code should be maintainable -- if another programmer comes along to fix my code, non-obvious behavior can hide bugs or make functional code appear buggy (see readability) "The right tool for the right job" -- I'm sure you can come up with contrived examples that could use lookahead, but I've never come across something that really needs them in my real-world development work. Is there anything that they're really the best tool for, as opposed to, say, multiple regexps (or, alternatively, are they the best tool for most cases they're used for today). My question is this: Is it good practice to use lookahead/lookbehind in regular expressions, or are they simply a hack that have found their way into modern production code? I'd be perfectly happy to be convinced that I'm wrong about this, and simple examples are useful for examples or illustration, but by themselves, won't be enough to convince me.

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  • C# regex: negative lookahead fails with the single line option

    - by Sylverdrag
    I am trying to figure out why a regex with negative look ahead fails when the "single line" option is turned on. Example (simplified): <source>Test 1</source> <source>Test 2</source> <target>Result 2</target> <source>Test 3</source> This: <source>(?!.*<source>)(.*?)</source>(?!\s*<target) will fail if the single line option is on, and will work if the single line option is off. For instance, this works (disables the single line option): (?-s:<source>(?!.*<source>)(.*?)</source>(?!\s*<target)) My understanding is that the single line mode simply allows the dot "." to match new lines, and I don't see why it would affect the expression above. Can anyone explain what I am missing here?

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  • grep with negative lookbehind

    - by Dan Fabulich
    I'm trying to grep through a bunch of files in nested subdirectories to look for regular expression matches; my regex requires negative lookbehind. Perl has negative lookbehind, but as far as I can tell GNU grep doesn't support negative lookbehinds. What's the easiest way to get an equivalent to GNU grep that supports negative lookbehinds? (I guess I could write my own mini-grep in Perl, but that doesn't seem like it should be necessary. My copy of the Perl Cookbook includes source for tcgrep; is that what I should use? If so, where's the latest version? Don't tell me I have to type this entire program!)

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  • LookAhead Regex in .Net - unexpected result

    - by AaronM
    Hello, I am a bit puzzled with my Regex results (and still trying to get my head around the syntax). I have been using http://regexpal.com/ to test out my expression, and its works as intended there, however in C# its not as expected. Here is a test - an expression of the following: (?=<open>).*?(?=</open>) on an input string of: <open>Text 1 </open>Text 2 <open>Text 3 </open>Text 4 <open>Text 5 </open> I would expect a result back of <open>Text1 <open>Text 2 <open>Text 3... etc However when I do this in C# it only returns the first match of <open>Text1 How do I get all five 'results' back from the Regex? Regex exx = new Regex("(?=<open>).*?(?=</open>)", RegexOptions.IgnoreCase | RegexOptions.Singleline); string input = "<open>Text 1</open> Text 2 <open> Text 3 </open> Text 4 <open> Text 5 </open>"; string result = Regex.Match(input, exx.ToString(), exx.Options).ToString();

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  • OCaml: Matching with any negative

    - by nicotine
    Is there a way to get pattern matching to match my value with any negative number? It does not matter what the negative number is I just need to match with any negative. I have accomplished what I want with this simple code: let y = if(n < 0) then 0 else n in match y with 0 -> [] | _ -> [x] @ clone x (n - 1) But I want to eliminate that if statement and just get it to check it as another case in the match statement

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  • Regex negative lookahead

    - by Alyn
    I need to modify this regex href=\"(.*)\" which matches this... href="./pothole_locator_map.aspx?lang=en-gb&lat=53.153977&lng=-3.533306" To NOT match this... href="./pothole_locator_map.aspx?lang=en-gb&lat=53.153977&lng=-3.533306&returnurl=AbandonedVehicles.aspx" Tried this, but with no luck href=\"(.*)\"(?!&returnurl=AbandonedVehicles.aspx) Any help would be much appreciated. Thanks, Al.

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  • Poor performance / speed of regex with lookahead

    - by Hugo Zaragoza
    I have been observing extremely slow execution times with expressions with several lookaheads. I suppose that this is due to underlying data structures, but it seems pretty extreme and I wonder if I do something wrong or if there are known work-arounds. The problem is determining if a set of words are present in a string, in any order. For example we want to find out if two terms "term1" AND "term2" are somewhere in a string. I do this with the expresion: (?=.*\bterm1\b)(?=.*\bterm2\b) But what I observe is that this is an order of magnitude slower than checking first just \bterm1\b and just then \bterm2\b This seems to indicate that I should use an array of patterns instead of a single pattern with lookaheads... is this right? it seems wrong... Here is an example test code and resulting times: public static void speedLookAhead() { Matcher m, m1, m2; boolean find; int its = 1000000; // create long non-matching string char[] str = new char[2000]; for (int i = 0; i < str.length; i++) { str[i] = 'x'; } String test = str.toString(); // First method: use one expression with lookaheads m = Pattern.compile("(?=.*\\bterm1\\b)(?=.*\\bterm2\\b)").matcher(test); long time = System.currentTimeMillis(); ; for (int i = 0; i < its; i++) { m.reset(test); find = m.find(); } time = System.currentTimeMillis() - time; System.out.println(time); // Second method: use two expressions and AND the results m1 = Pattern.compile("\\bterm1\\b").matcher(test); m2 = Pattern.compile("\\bterm2\\b").matcher(test); time = System.currentTimeMillis(); ; for (int i = 0; i < its; i++) { m1.reset(test); m2.reset(test); find = m1.find() && m2.find(); } time = System.currentTimeMillis() - time; System.out.println(time); } This outputs in my computer: 1754 150

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  • FileInputStream negative skip

    - by Peter Štibraný
    I'm trying to find more about history of FileInputStream.skip(negative) operation. According to InputStream documentation: If n is negative, no bytes are skipped. It seems that implementation of FileInputStream from Sun used to throw IOException instead, which is now also documented in Javadoc: If n is negative, an IOException is thrown, even though the skip method of the InputStream superclass does nothing in this case. I just tried that, and found that FileInputStream.skip(-10) did in fact return -10! It didn't threw exception, it didn't even return 0, it returned -10. (I've tried with Java 1.5.0_22 from Sun, and Java 1.6.0_18 from Sun). Is this a known bug? Why hasn't it been fixed, or why documentation is kept the way it is? Can someone point me to some discussion about this issue? I can't find anything.

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  • Function Returning Negative Value

    - by Geowil
    I still have not run it through enough tests however for some reason, using certain non-negative values, this function will sometimes pass back a negative value. I have done a lot of manual testing in calculator with different values but I have yet to have it display this same behavior. I was wondering if someone would take a look at see if I am missing something. float calcPop(int popRand1, int popRand2, int popRand3, float pERand, float pSRand) { return ((((((23000 * popRand1) * popRand2) * pERand) * pSRand) * popRand3) / 8); } The variables are all contain randomly generated values: popRand1: between 1 and 30 popRand2: between 10 and 30 popRand3: between 50 and 100 pSRand: between 1 and 1000 pERand: between 1.0f and 5500.0f which is then multiplied by 0.001f before being passed to the function above Edit: Alright so after following the execution a bit more closely it is not the fault of this function directly. It produces an infinitely positive float which then flips negative when I use this code later on: pPMax = (int)pPStore; pPStore is a float that holds popCalc's return. So the question now is, how do I stop the formula from doing this? Testing even with very high values in Calculator has never displayed this behavior. Is there something in how the compiler processes the order of operations that is causing this or are my values simply just going too high? If the later I could just increase the division to 16 I think.

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  • PHP profiling with microtime(): Negative time?

    - by Boldewyn
    For a very simple profiling I use microtime() like this: $now = microtime(); for (...) { // do something echo microtime() - $now; $now = microtime(); } Now, the output of the echo line seems completely random, that is, I expected fluctuations, but I don't expected negative numbers showing up. However, a typical result contains ~ 1/3 negative numbers. I confirmed this on Solaris (PHP 5.0.x) and WinVista (PHP 5.2.3). What the heck is going on here? Have I invented accidently a time machine?

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  • How to read in a negative double with scanf() in C

    - by rize
    I'm learning basics of C and writing a simple first order equation solver. I want the input to be exactly ax+b=c or ax-b=c, where a, b, c are double type. I'm employing scanf() to read in user input and to check if it's of the correct form. However, if I enter a negative double, -4.6 say, as the "a" in the equation, scanf() won't read the a,b,c correctly. I'm using %lf inside scanf(). How do I read a negative double, then? Many thanks. My code: if (scanf("%lfx+%lf=%lf", &a, &b, &c)) more code If I use as the input "-6.2x+3.4=-5.9", the value 3.4 will be assinged to variable a, while b and c remain as they were and "more code" is run.

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  • Perl: Negative look behind regex question [migrated]

    - by James
    The Perlre in Perldoc didn't go into much detail on negative look around but I tried testing it, and didn't work as expected. I want to see if I can differentiate a C preprocessor macro definition (e.g. #define MAX(X) ....) from actual usage (y = MAX(x);), but it didn't work as expected. my $macroName = 'MAX'; my $macroCall = "y = MAX(X);"; my $macroDef = "# define MAX(X)"; my $boundary = qr{\b$macroName\b}; my $bstr = " MAX(X)"; if($bstr =~ /$boundary/) { print "boundary: $bstr matches: $boundary\n"; } else { print "Error: no match: boundary: $bstr, $boundary\n"; } my $negLookBehind = qr{(?<!define)\b$macroName\b}; if($macroCall =~ /$negLookBehind/) # "y = MAX(X)" matches "(?<!define)\bMAX\b" { print "negative look behind: $macroCall matches: $negLookBehind\n"; } else { print "no match: negative look behind: $macroCall, $negLookBehind\n"; } if($macroDef =~ /$negLookBehind/) # "#define MAX(X)" should not match "(?<!define)\bMAX\b" { print "Error: negative look behind: $macroDef matches: $negLookBehind\n"; } else { print "no match: negative look behind: $macroDef, $negLookBehind\n"; } It seems that both $macroDef and $macroCall seem to match regex /(?<!define)\b$macroName\b/. I backed off from the original /(?<\#)\s*(?<!define)\b$macroName\b/ since that didn't work either. So what did I screw up? Also does Perl allow chaining of multiple look around expressions?

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  • RegExp: want to find all links that do not end in ".html"

    - by grovel
    Hi, I'm a relative novice to regular expressions (although I've used them many times successfully). I want to find all links in a document that do not end in ".html" The regular expression I came up with is: href=\"([^"]*)(?<!html)\" In Notepad++, my editor, href=\"([^"]*)\" finds all the links (both those that end in "html" and those that do not). Why doesn't negative lookbehind work? I've also tried lookahead: href=\"[^"]*(?!html\") but that didn't work either. Can anybody help? Cheers, grovel

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  • Regex negative look-behind in hgignore file

    - by jco
    I'm looking for a way to modify my .hgignore file to ignore all "Properties/AssemblyInfo.cs" files except those in either the "Test/" or the "Tests/" subfolders. I tried using the negative look-behind expression (?<!Test)/Properties/AssemblyInfo\.cs$, but I didn't find a way to "un-ignore" in both folders "Test/" and "Tests/".

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  • Why does multiplying a double by -1 not give the negative of the current answer

    - by Ankur
    I am trying to multiply a double value by -1 to get the negative value. It continues to give me a positive value double man = Double.parseDouble(mantissa); double exp; if(sign.equals("plus")){ exp = Double.parseDouble(exponent); } else { exp = Double.parseDouble(exponent); exp = exp*-1; } System.out.println(man+" - "+sign+" - "+exp); The printed result is 13.93 - minus - 2.0 which is correct except that 2.0 should be -2.0

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  • Negative numbers, arrays javascript!

    - by zizzamia
    I was implementing a routing algorithm in javascript, but when I assign a negative one variable in the array gives me this error: invalid array length. var node = new Array() node[0] = new Array(6,7) node[1] = new Array(5,-4,8) node[2] = new Array(-2) //Here, invalid array length I do not know how to resolve this error.

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  • CSS Negative margins for positioning.

    - by Kyle Sevenoaks
    Is it ok to use negative margins for positioning? I have a lot in my current site and feel like it's not such a stable way to position things. I usually suggest to use them too. For example I have a checkout page with three divs on top of each other <div class="A"> header </div> <div class="B"> content </div> <div class="C"> footer </div> (A, B and C), which are meant to sit on top of each other, to appear attached. I did this using: .B { margin-top: -20px; } On div B, to meet the bottom of div A. Is this good practice or shall I re-code using top and left?

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  • Using the mpz_powm functions from the GMP/MPIR libraries with negative exponents

    - by Mihai Todor
    Please consider the following code: mpz_t x, n, out; mpz_init_set_ui(x, 2UL); mpz_init_set_ui(n, 7UL); mpz_init(out); mpz_invert(out, x, n); gmp_printf ("%Zd\n", out);//prints 4. 2 * 4 (mod 7) = 1. OK mpz_powm_ui(out, x, -1UL, n);//prints 1. 2 * 1 (mod 7) = 2. How come? gmp_printf ("%Zd\n", out); mpz_clear(x); mpz_clear(n); mpz_clear(out); I am unable to understand how the mpz_powm functions handle negative exponents, although, according to the documentation, it is supposed to support them. I would expect that raising a number to -1 modulo n is equivalent to inverting it modulo n. What am I missing here?

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  • Feedback Filtration&ndash;Processing Negative Comments for Positive Gains

    - by D'Arcy Lussier
    After doing 7 conferences, 5 code camps, and countless user group events, I feel that this is a post I need to write. I actually toyed with other names for this post, however those names would just lend itself to the type of behaviour I want people to avoid – the reactionary, emotional response that speaks to some deeper issue beyond immediate facts and context. Humans are incredibly complex creatures. We’re also emotional, which serves us well in certain situations but can hinder us in others. Those of us in leadership build up a thick skin because we tend to encounter those reactionary, emotional responses more often, and we’re held to a higher standard because of our positions. While we could react with emotion ourselves, as the saying goes – fighting fire with fire just makes a bigger fire. So in this post I’ll share my thought process for dealing with negative feedback/comments and how you can still get value from them. The Thought Process Let’s take a real-world example. This week I held the Prairie IT Pro & Dev Con event. We’ve gotten a lot of session feedback already, most of it overwhelmingly positive. But some not so much – and some to an extreme I rarely see but isn’t entirely surprising to me. So here’s the example from a person we’ll refer to as Mr. Horrible: How was the speaker? Horrible! Worst speaker ever! Did the session meet your expectations? Hard to tell, speaker ruined it. Other Comments: DO NOT bring this speaker back! He was at this conference last year and I hoped enough negative feedback would have taught you to not bring him back...obviously not...I will not return to this conference next year if this speaker is brought back. Now those are very strong words. “Worst speaker ever!” “Speaker ruined it” “I will not return to this conference next year if the speaker is brought back”. The speakers I invite to speak at my conference are not just presenters but friends and colleagues. When I see this, my initial reaction is of course very emotional: I get defensive, I get angry, I get offended. So that’s where the process kicks in. Step 1 – Take a Deep Breath Take a deep breath, calm down, and walk away from the keyboard. I didn’t do that recently during an email convo between some colleagues and it ended up in my reacting emotionally on Twitter – did I mention those colleagues follow my Twitter feed? Yes, I ate some crow. Ok, now that we’re calm, let’s move on to step 2. Step 2 – Strip off the Emotion We need to take off the emotion that people wrap their words in and identify the root issues. For instance, if I see: “I hated this session, the presenter was horrible! He spoke so fast I couldn’t make out what he was saying!” then I drop off the personal emoting (“I hated…”) and the personal attack (“the presenter was horrible”) and focus on the real issue this person had – that the speaker was talking too fast. Now we have a root cause of the displeasure. However, we’re also dealing with humans who are all very different. Before I call up the speaker to talk about his speaking pace, I need to do some other things first. Back to our Mr. Horrible example, I don’t really have much to go on. There’s no details of how the speaker “ruined” the session or why he’s the “worst speaker ever”. In this case, the next step is crucial. Step 3 – Validate the Feedback When I tell people that we really like getting feedback for the sessions, I really really mean it. Not just because we want to hear what individuals have to say but also because we want to know what the group thought. When a piece of negative feedback comes in, I validate it against the group. So with the speaker Mr. Horrible commented on, I go to the feedback and look at other people’s responses: 2 x Excellent 1 x Alright 1 x Not Great 1 x Horrible (our feedback guy) That’s interesting, it’s a bit all over the board. If we look at the comments more we find that the people who rated the speaker excellent liked the presentation style and found the content valuable. The one guy who said “Not Great” even commented that there wasn’t anything really wrong with the presentation, he just wasn’t excited about it. In that light, I can try to make a few assumptions: - Mr. Horrible didn’t like the speakers presentation style - Mr. Horrible was expecting something else that wasn’t communicated properly in the session description - Mr. Horrible, for whatever reason, just didn’t like this presenter Now if the feedback was overwhelmingly negative, there’s a different pattern – one that validates the negative feedback. Regardless, I never take something at face value. Even if I see really good feedback, I never get too happy until I see that there’s a group trend towards the positive. Step 4 – Action Plan Once I’ve validated the feedback, then I need to come up with an action plan around it. Let’s go back to the other example I gave – the one with the speaker going too fast. I went and looked at the feedback and sure enough, other people commented that the speaker had spoken too quickly. Now I can go back to the speaker and let him know so he can get better. But what if nobody else complained about it? I’d still mention it to the speaker, but obviously one person’s opinion needs to be weighed as such. When we did PrDC Winnipeg in 2011, I surveyed the attendees about the food. Everyone raved about it…except one person. Am I going to change the menu next time for that one person while everyone else loved it? Of course not. There’s a saying – A sure way to fail is to try to please everyone. Let’s look at the Mr. Horrible example. What can I communicate to the speaker with such limited information provided in the feedback from Mr. Horrible? Well looking at the groups feedback, I can make a few suggestions: - Ensure that people understand in the session description the style of the talk - Ensure that people understand the level of detail/complexity of the talk and what prerequisite knowledge they should have I’m looking at it as possibly Mr. Horrible assumed a much more advanced talk and was disappointed, while the positive feedback by people who – from their comments – suggested this was all new to them, were thrilled with the session level. Step 5 – Follow Up For some feedback, I follow up personally. Especially with negative or constructive feedback, its important to let the person know you heard them and are making changes because of their comments. Even if their comments were emotionally charged and overtly negative, it’s still important to reach out personally and professionally. When you remove the emotion, negative comments can be the best feedback you get. Also, people have bad days. We’ve all had one of “those days” where we talked more sternly than normal to someone, or got angry at something we’d normally shrug off. We have various stresses in our lives and sometimes they seep out in odd ways. I always try to give some benefit of the doubt, and re-evaluate my view of the person after they’ve responded to my communication. But, there is such a thing as garbage feedback. What Mr. Horrible wrote is garbage. It’s mean spirited. It’s hateful. It provides nothing constructive at all. And a tell-tale sign that feedback is garbage – the person didn’t leave their name even though there was a field for it. Step 6 – Delete It Feedback must be processed in its raw form, and the end products should drive improvements. But once you’ve figured out what those things are, you shouldn’t leave raw feedback lying around. They are snapshots in time that taken alone can be damaging. Also, you should never rest on past praise. In a future blog post, I’m going to talk about how we can provide great feedback that, even when its critical, can still be constructive.

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  • How to get lookahead symbol when constructing LR(1) NFA for parser?

    - by greenoldman
    I am reading an explanation (awesome "Parsing Techniques" by D.Grune and C.J.H.Jacobs; p.292 in the 2nd edition) about how to construct an LR(1) parser, and I am at the stage of building the initial NFA. What I don't understand is how to get/compute a lookahead symbol. Here is the example from the book, the grammar: S -> E E -> E - T E -> T T -> ( E ) T -> n n is terminal. The "weird" transitions for me are is the sequence: 1) S -> . E eof 2) E -> . E - T eof 3) E -> . E - T - 4) E -> E . - T - 5) E -> E - . T - (Note: In the above table, the state numbers are in front and the lookahead symbol is at the end.) What puzzles me is that transition from (4) to (5) means reading - token, right? So how is it that - is still a lookahead symbol and even more important why is it that eof is no longer a lookahead symbol? After all in an input such as n - n eof there is only one - symbol. My naive thinking tells me (5) should be written as: 5) E -> E - . T - eof And another thing -- n is terminal. Why it is not used at all as a lookahead symbol? I mean -- we expect to see - or (, it is ok, but lack of n means we are sure it won't appear in input? Update: after more reading I am only more confused ;-) I.e. what is really a lookahead? Because I see such state as (p.292, 2nd column, 2nd row): E -> E . - T eof Lookahead says eof but the incoming input says -. Isn't it a contradiction? And it is not only in this book.

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