![]() When our programming language returns the results, we ignore the overall matches (that's the trash bin) and instead turn our whole attention to Group 1 matches, which contain what we were after. The trick is that we match what we don't want on the left side of the alternation (the |), then we capture what we do want on the right side. Well, you'll now see how simple the problem becomes when you use the best regex trick ever: Note that while we try to match unwanted content, we swallow entire sets of )+*?(?:$|)) In other words, we want to exclude "Tarzan".Īt first you may think of framing Tarzan between a negative lookbehind and a negative lookahead: Simple Case: fixed-width non-match, as in "Tarzan"įirst, let's examine a "simple case": we want to match Tarzan except when this exact word is in double-quotes. To see how convenient the trick is, it helps to first see how inconvenient some matching tasks can be when you don't know it. Will you do me a favor and report any typos or bugs you find? Thanks! It's sure to have typos and perhaps bugs. But if you don't care about the typical solutions to the problems addressed by the technique, you can skip directly to the description of the trick. For full potency, I recommend you read the whole article in sequence. ![]() Once you grasp the technique, you will see that under a certain light, these are all nearly the same question.įor convenience, here are some jumping points. ✽ How do I ignore all content that is bolded (… and other contexts)? ✽ How do I match every word except those on a blacklist (or other contexts)? ✽ How do I match xyz except in contexts a, b or c? ✽ How do I match a word unless it's surrounded by quotes? Here are some of the questions that our regex trick is able to answer with speed and grace: No need to buckle up, the technique itself is delightfully simple.Įxcluding certain Contexts while Matching or Replacing ![]() In a typical context that is no problem, but if you are working with an enormous file, the trash can may get so large that you could run into memory issues. The regex engine dumps unwanted content into a trash can. Code samples for the six typical situations are provided below. ![]() ✽ The point above also means that you may have to write one or two extra lines of code, but that is a light price to pay for a much cleaner, lighter and easier to maintain regex. ✽ It relies on your ability to inspect Group 1 captures (at least in the generic flavor), so it will not work in a non-programming environment, such as a text editor's search-and-replace function or a grep command. ✽ It will not make small talk with your mother-in-law. ✽ It will not butter the reverse side of a toast. At least, until now.īefore we proceed, I should point out some limitations of the technique: ✽ It is usually more efficient than competing methods. ✽ It is portable over numerous regex flavors. ✽ It is easy to extend when requirements change. ✽ It is simple to implement in most programming languages. ✽ These questions are ones that even competent regex coders often have trouble answering gracefully. ✽ It answers not one, but several common and practical regex questions. In contrast, the reason I drum up the technique on this page as the "best regex trick ever" is that it has several properties: But however clever these tricks, I would not call any of them the "best regex trick ever", for the simple reason that they are one-off techniques with limited scope. With regex there's always more to learn, and there's always a more clever person than you (unless you're the lone guy sitting on top of the mountain), so I've often been exposed to awesome tricks that were out of my league-for instance the famous regex to validate that a number is prime, or some fiendish uses of recursion. A regex trick uses regex grammar to compose a "phrase" that achieves certain goals. In contrast, a "trick" is not a single point of syntax such as a negated character class or a lazy quantifier. They are neat, to be sure, but they are how regex works, and nothing more. However, as you mature as a regex practitioner, you come to regard these techniques for what they are: language features rather than tricks. At other points in your career, you'll surely fall in love with regex bits such as + to match all the content between certain delimiters (in this case double quotes), or with atomic groups. *? prevents you from steamrolling from the start to the end of a string such as Tarzan likes Jane may seem like the best regex trick ever. When you start out with regex, learning that the lazy question mark in. ![]() I'll concede right away that deciding what constitutes the best technique in any field is a curly matter. So you're doubtful at the mention of a "best regex trick"?įine. ![]()
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