Recursive Regular Expressions. The regular expressions we use in our daily lives are actually not that "regular. " Most of the languages support some kind of extended regular expressions that are computationally more powerful than the "regular" regular expressions as defined by the formal language theory. For instance, the so often used capture buffers add auxiliary storage to the regular expressions that allow them to match an arbitrary pattern repeatedly. Or look-ahead assertions that allow the regular expression engine to peek ahead before it making a decision. These extensions make regular expressions powerful enough to describe some context-free grammars. The Perl programming language has an especially rich with regex engine. This allows us to construct something really interesting - we can define a regular expression that has itself in the "code" part.
Here is a Perl regular expression that matches 0n1n: $regex = qr/0(?? This regular expression matches a 0 followed by itself zero or one time, followed by a one. . #! Gruber's URL Regular Expression Explained. While America threw on its eating pants and combed the Thursday circulars for deals, John Gruber spent Thanksgiving preparing to unveil his regular expression for finding URLs in arbitrary text. \b(([\w-]+://? |www[.]) [^\s()<>]+(? :\([\w\d]+\)|([^[:punct:]\s]|/))) Pretty dense. Let’s be that guy and break it out, /x style (ignoring white space, with comments) \b #start with a word boundary ( #<capture_1> ( #<capture_2> [\w-]+://? It’s a great start for a hard problem. While John accounted for Wikipedia’s weird parenthesis in URLs, he didn’t account for double parentheses, such as won’t capture URLs that lack both a protocol and a www.
Neither is an easy problem, but the last 10% of any regular expression based solution is always the hardest. \([\w\d]+\) #handles weird parenthesis in URLs (#handles weird parenthesis in URLs ( with this (? This still wouldn’t handle something like. The Treacherous Optimization. The Treacherous Optimization May 30th, 2006 Old age and treachery will beat youth and skill every time. "I'm going to beat grep by thirty percent! " I confidently crow to anyone who would listen, those foolish enough to enter my office. And my girlfriend too, who's contractually obligated to pay attention to everything I say. See, I was working on Hex Fiend, and searching was dog slow. The first step in any potentially impossible project is, of course, to announce that you are on the verge of succeeding. I imagine the author of grep, Ultimate Unix Geek, squinting at vi; the glow of a dozen xterms is the only light to fall on his ample frame covered by overalls, cheese doodles, and a tangle of beard.
String searching Having exhausted all my trash-talking avenues, it's time to get to work. Boyer-Moore works like this: you have some string you're looking for, which we'll call the needle, and some string you want to find it in, which we'll call the haystack. I get shot down Ouch. "You bastard! " RegEx: online regular expression testing. Sregex - Project Hosting on Google Code. The sregex module implements Structural Regular Expressions. Structural Regular Expressions were created by Rob Pike and covered in this paper: Structural regular expressions work by describing the shape of the whole string, not just the piece you want to match. Each pattern is a list of operators to perform on a string, each time constraining the range of text that matches the pattern. Examples will make this much clearer. The first operator to consider is the x// operator, which means e(x)tract. Given the source string "Atom-Powered Robots Run Amok" and the pattern "x/A...
/" the result would be ['Atom', 'Amok']. >>> list(sres("Atom-Powered Robots Run Amok", "x/A... /")) ['Atom', 'Amok'] A pattern can contain mulitple operators, separated by whitespace, which are applied in order, each to the result of the previous match. >>> list(sres("Atom-Powered Robots Run Amok", "x/A.../ x/.
There are four operators in total: Installation. HiFi RegExp Tool - Write and Test Regular Expressions in Real-Ti. Rue. Email regex? Headache relief for programmers :: regular expression generator. Help me find my lost, online, visual regular-expression builder. Regular Expressions for Regular Programmers. JRX: real-time JavaScript RegExp evaluator. The implementation of Factor's regexp library. I've been working on Factor's regular expression library, initially written by Doug Coleman, for the past few weeks.
Recently, the library became good enough that I've pushed it to Factor's main repository. The latest Factor binaries have this new library. The library uses an standard algorithm of converting a regular expression into an NFA, and that into a DFA which can be executed. This is a tradeoff: the code generated will be faster than you would get from a backtracking search or an NFA interpreter, but it takes exponential time, in the worst case, to generate the DFA.
I might revisit this later. The main features missing now are Possessive and reluctant matchingGroup captureUnicode support, in the form of UTS 18 level 1 compliance with some level 2 featuresRight now, I'm working on Unicode support. The rest of this article is an overview of how the regexp engine works. The parser The parser is implemented with Chris Double's packrat parsing library. Constructing an NFA Disambiguation. Clojure’s new regex syntax. Last week, Rich Hickey announced a few notable changes to Clojure, including ahead-of-time compilation and a cleaner syntax for regular expressions. Both are improvements, but the syntax is especially interesting for a reason unrelated to its function. First, a quick overview. 1. What has changed In a sentence, fewer backslashes. The notation is now more in line with that of scripting languages, where regular expressions are first-class literals, than that of general-purpose languages like C++ or Java, where regexes are just specialized strings.
Say we are given a stream including this text: ... We want to select IMG tags and capture the basename (without extension) of each source file. <img [whitespace]+ src=" [word-char]+ / [digit]+ / ([word-char]+) ... Converting this to Clojure’s old syntax gives us a somewhat unwieldly #"<img\\s+src=\"\\w+/\\d+/(\\w+)". (let [lines "...
The new update to the reader allows us to remove the double escaping of the regex specials in the literal: 2. Java #? Learning to Use Regular Expressions. Often if you find that your regular expressions are matching too much, a useful procedure is to reformulate the problem in your mind. Rather than thinking about "what am I trying to match later in the expression? " ask yourself "what do I need to avoid matching in the next part? " Often this leads to more parsimonious pattern matches. Often the way to avoid a pattern is to use the complement operator and a character class. The trick here is that there are two different ways of formulating almost the same sequence. For people who have thought about basic probability, the same pattern occurs. Flex 3 Regular Expression Explorer. Online js Regular Expression Tester. Feel free to test JavaScript's RegExp support right here in your browser.
Obviously, JavaScript (or Microsoft's variant JScript) will need to be enabled in your browser for this to work. Since this tester is implemented in JavaScript, it will reflect the features and limitations of your web browser's JavaScript implementation. If you're looking for a general-purpose regular expression tester supporting a variety of regex flavors, grab yourself a copy of RegexBuddy. Regexp. 5 Regular Expressions Every Web Programmer Should Know.
Regular Expressions: Now You Have Two Problems. I love regular expressions. No, I'm not sure you understand: I really love regular expressions. You may find it a little odd that a hack who grew up using a language with the ain't keyword would fall so head over heels in love with something as obtuse and arcane as regular expressions.
I'm not sure how that works. But it does. Regular expressions rock.They should absolutely be a key part of every modern coder's toolkit. If you've ever talked about regular expressions with another programmer, you've invariably heard this 1997 chestnut: Some people, when confronted with a problem, think "I know, I'll use regular expressions. " The quote is from Jamie Zawinski, a world class hacker who I admire greatly. Perl's nature encourages the use of regular expressions almost to the exclusion of all other techniques; they are far and away the most "obvious" (at least, to people who don't know any better) way to get from point A to point B.
The first quote is too glib to be taken seriously. But wait! SLRE - Super Light Regular Expression library. Parsing Strings With jQuery : DevKick Blog. Parse - cruiser: js parser generator. When you reach the limits of regular expressions in JavaScript, you have two choices: 1. Realize that what you are trying to do is probably not a good idea. 2. Write yourself a parser generator and keep on going. The addition of CSS 3 selector support broke our regex-based parser for Behaviors. What this library gives you is the ability to write parsers very easily. Here's the grammar for CSS (probably not perfect, but good enough for our purposes): Most of it is pretty self-explanatory. Functions like pair and between give the parser some additional intelligence beyond just sequences of tokens and alternate paths.
Each of the functions described below return a function that will match based on the given criteria. Token( token ). Any( rule1, rule2, ... ). Each( rule1, rule2, ...). Many( rule ). List( rule, delim, trailing ). Between( rdelim, rule, ldelim ). Pair( rule1, rule2, delim ). Process( rule, fn ). Ignore( rule). You can easily add additional operators. A Regular Expression Test Applet. The point of this applet is to provide a convenient means of testing regular expressions before embedding them in a Java program. The buttons represent calls to the Java methods with the same name. The applet executes (approximately) the following code: pattern = Pattern.compile(Contents of the Pattern: field); matcher = pattern.matcher(Contents of the String: field); matcher.Whatever_method_you_clicked(); Display results; The In Java: field shows how you would have to write the regular expression as a Java literal string. You cannot type into this area, but you can copy from it and paste the literal string into your Java programs.
For successful matches, the Results: area will show what values you will get if you call the methods start(), end(), and group(n). For those who are interested, here's the source code. Regular Expression Online Tester. Extreme regex foo: what you need to know to become a regular exp. Regular Expression Matching Can Be Simple And Fast. Russ Coxrsc@swtch.com January 2007 Introduction This is a tale of two approaches to regular expression matching. One of them is in widespread use in the standard interpreters for many languages, including Perl. The other is used only in a few places, notably most implementations of awk and grep.
The two approaches have wildly different performance characteristics: Let's use superscripts to denote string repetition, so that a? Notice that Perl requires over sixty seconds to match a 29-character string. It may be hard to believe the graphs: perhaps you've used Perl, and it never seemed like regular expression matching was particularly slow. Historically, regular expressions are one of computer science's shining examples of how using good theory leads to good programs. Today, regular expressions have also become a shining example of how ignoring good theory leads to bad programs. Regular Expressions Regular expressions are a notation for describing sets of character strings. Finite Automata.