Regex & Facebook Experience Tech Talk

I recently gave a talk for a group of university students studying web development in Belgrade, Serbia. In it, I talked about my background, my experience starting in tech and eventually working at Facebook for seven years, and (most relevant to this blog) I spent about 20 minutes showing off some cool and advanced regular expressions. Specifically, regexes for switching Fahrenheit to Celsius, deleting repeated words, deleting non-adjacent duplicate lines, checking password complexity, reformatting names, adding thousands separators, matching balanced parentheses, and matching palindromes.

You can find the video and slides here (Lecture at SAE Belgrade: Lessons from Facebook) or you can watch the video on YouTube (with the regex portion starting at 52:26).

XRegExp 3.0.0!

After 3+ years, XRegExp 3.0.0 has been released. Standout features are dramatically better performance (many common operations are 2x to 50x faster) and support for full 21-bit Unicode (thanks to Mathias Bynens). I’ve also just finished updating all the documentation on so go check that out. 🙂

If you haven’t used XRegExp before, it’s an MIT licensed JavaScript library that provides augmented (and extensible!) regular expressions. You get new modern syntax and flags beyond what browsers support natively. XRegExp is also a regex utility belt with tools to make your client-side grepping and parsing easier, while freeing you from worrying about pesky cross-browser inconsistencies and things like manually manipulating lastIndex or slicing strings when tokenizing.

Version 3.0.0 has lots of additional features, options, fine tuning, cross-browser fixes, some new simplified syntax, and thousands of new tests. And it still supports all the browsers. Check out the long list of changes. There are a few minor breaking changes that shouldn’t affect most people and have easy workarounds. I’ve listed them all below, but see the full changelog if you need more details about them.

  • XRegExp.forEach no longer accepts or returns its context. Use binding with the provided callback instead.
  • Moved character data for Unicode category L (Letter) from Unicode Base to Unicode Categories. This has no effect if you’re already using Unicode Categories or XRegExp-All.
  • Using the same name for multiple named capturing groups in a single regex is now a SyntaxError.
  • Removed the 'all' shortcut used by XRegExp.install/uninstall.
  • Removed the Prototypes addon, which added methods apply, call, forEach, globalize, xexec, and xtest to XRegExp.prototype. These were all just aliases of methods on the XRegExp object.
  • A few changes affect custom addons only: changed the format for providing custom Unicode data, replaced XRegExp.addToken’s trigger and customFlags options with new flag and optionalFlags options, and removed the this.hasFlag function previously available within token definition functions.

You can download the new release on GitHub or install via npm. I’d love to hear feedback and common regex-related use cases that you think could be simplified via new XRegExp features. Let me know here or in GitHub issues. Thanks!

Regex Cookbook 2nd Edition Contest Winners

For the last six days, I've been running a contest on my blog to win one of 15 free copies of Regular Expressions Cookbook, 2nd Edition. Thanks to all who participated and spread the word! I've read every one of your comments, and appreciate the congratulations and the great comments about the first edition, etc. There's a lot of love for regular expressions out there!

Of the almost 400 who requested a copy, people specifically mentioned that they wanted an ebook rather than a print copy at a ratio of about 0.7 to 1, which was a lot higher than I expected. That bodes well for Regular Expressions Cookbook, though, considering that the first edition was O'Reilly Media's bestselling ebook of 2010.

Without further ado, the randomly-selected contest winners are (drumroll)…

  1. Travis Hardiman
  2. Tony Garcia
  3. Jennifer Lumer
  4. Bessie Chan
  5. Derek Brigner
  6. Marco Antonio
  7. Kent Isaly
  8. Marcus Barnes
  9. Kennie Cruz
  10. Jacob Christiansen
  11. Stafleu Buitendag
  12. Michael Mongeau
  13. Rod Vagg
  14. Henk Scholten
  15. Wim Mostmans

I've contacted all of the winners by email. If you haven't already heard from me, check your spam folder.

I've additionally chosen two people to send signed copies to: Edward Beckett and Drew Bennett. 🙂

For those who want to buy a copy, you can get it directly from O'Reilly Media, where it's on sale until September 18th, using code B2S2. O'Reilly offers DRM-free ebook copies in PDF, Mobi, and ePub formats. The book is also available with a discount and free shipping from Amazon:

Amazon also sells ebook copies for Kindle.

Congratulations to all winners, and thanks again to everyone else for participating! If you want to know more about the book, check out Rob Friesel Jr's detailed review of the second edition.

Win a Free Copy of Regex Cookbook 2nd Edition

Update: This contest is now finished. See the list of winners.

I'm excited to announce the release of Regular Expressions Cookbook 2nd Edition, which I wrote together with regex superguru Jan Goyvaerts. It has actually been available as an ebook for a couple weeks on, but as of now, it is also in stock on

To promote this release, O'Reilly Media is giving away free copies to 15 people who comment on this post on or before September 7th! To get a free copy, you must read the details at the end of this post. But first, some FAQs about the second edition:

Wait…this is a cookbook?

The book tackles hundreds of real-world regular expression tasks in a problem, solution, discussion format. There's also a detailed regular expression tutorial included, in the same format. Check out Jeff Atwood's and Ben Nadel's reviews of the first edition, which have more details about this.

Update: Rob Friesel Jr. posted an awesome, detailed review of the second edition.

The first edition was a bestseller, and is now available in eight languages. It briefly held Amazon's #1 spot for computer books upon its release in mid 2009, and the ebook version was O'Reilly's top seller of 2010.

What has changed with this new edition?

The second edition adds more content, and updates existing chapters. There are innumerable improvements, including the most noticeable addition of a new chapter written by Jan, titled Source Code and Log Files, and various new recipes interspersed with four of the other chapters.

There are 101 new pages in the second edition, and that's after shortening and removing some content from the first edition. There were 125 recipes in the first edition, upped to 146 in the second. Note that many of the book's recipes provide solutions and in-depth discussions for more than one problem. Tons of changes, ranging from minor copyedits and errata corrections to major revisions and the addition of significant new content, were made throughout the existing content. Everything was brought up to date with the latest standards, tools, and programming language versions. In particular, updates to Java and Perl since the first edition brought very significant regular expression changes. Plus, we've covered some advanced regular expression features that already existed the last time around, but didn't make it into the first edition.

The first edition was already groundbreaking for the depth of its explanations and its equal coverage of all regexes in eight programming languages (C#, Java, JavaScript, Perl, PHP, Python, Ruby, and VB.NET). The second edition significantly improves upon this by providing even more details about the many peculiarities and differences of the APIs, syntax, and behavior of these regex flavors and programming languages. The second edition also adds coverage of XRegExp when it provides a better solution than native JavaScript. IMO, the second edition is easily the most comprehensive source of information about the use of modern regexes across multiple programming languages—far more detailed than anything else in print or online.

What will interest long-term fans the most?

Even though there are lots of important changes throughout the book, the new recipes and the updated coverage for the latest programming languages are probably the main reasons for owners of the first edition to upgrade. The fully-new recipes cover creating a regex-based parser, validating password complexity, adding thousands separators to numbers, matching various kinds of numbers, decoding XML entities, and everything in the new Source Code and Log Files chapter. The coverage of XRegExp is also completely new in the second edition.

What will cause readers to trip over themselves in their haste to buy a copy?

If you read though the book, you'll learn about a lot of things—much more than just regular expressions. You'll almost certainly learn something new about Unicode, phone numbers, and XML, as just a few examples. You'll learn that the eighth floor of the Saks Fifth Avenue store in New York City has its own ZIP code, which also happens to be the only ZIP code that includes letters. You'll learn that the Chicago Manual of Style and Merriam-Webster's Biographical Dictionary disagree on the correct alphabetical listing of the name Charles de Gaulle (my girlfriend and I are in opposing camps). Jan and I put a ton of research into the book, and we pay attention to details. I think that shines through.

Oh yeah, and along the way, you'll also become a Master Chef of regular expressions, able to slice and dice text with the best of them. But not everyone will want to actually read through the book. Some readers will prefer to take advantage of the cookbook format and read only the parts that solve their immediate problems. That's fine, too.

Many developers complain that they're continually relearning regular expressions, going back to the reference documents every time they need to write a new regex. The problem/solution approach of Regular Expressions Cookbook means you learn by doing, and we think that helps the details stick with you more securely than with the other books and websites out there.

Many regex novices turn to Google to get prewritten regexes that solve their problems. Unfortunately, if you're not already fluent in regex, you won't realize that 90% of the regexes out there have some kind of problem, be it returning false positives or negatives, performing inefficiently (or maybe even crashing your server when fed malicious data), being more complicated than necessary, not being portable, or what have you. When you use the regexes in Regular Expressions Cookbook, not only do you get detailed coverage of all the related issues (which helps you customize the solution, if necessary), you also get the peace of mind that you're using proven solutions by real subject-matter experts.

So how do I enter to win a free copy?

Simply comment on this blog post on or before 11:59 PM EDT on September 7th, and you'll be in the running. I wish I could leave the contest open a bit longer, but I'll be moving to California to work for a little Internet startup. You'll need to use your actual first and last name and email address with your comment. Names are published, but email addresses are not. Each person commenting has only one chance to win, regardless of how many comments they post. If you don't know what to write in your comment, just mention whether you'd prefer a print or ebook copy.

Shortly after this contest ends, I'll randomly choose 15 winners and contact them by email. If you prefer a printed copy, I'll be asking for your address and, if you're outside of the U.S., your phone number. O'Reilly will pay for shipping to anywhere in the world. Good luck!

Coauthor Jan Goyvaerts has written up his own summary of the changes in What's New in The Second Edition of Regular Expressions Cookbook.

Follow me on Twitter @slevithan or on GitHub at slevithan.

JavaScript Regex Lookbehind Redux

Five years ago I posted Mimicking Lookbehind in JavaScript on this blog, wherein I detailed several ways to emulate positive and negative lookbehind in JavaScript. My approaches back then were all fairly rough, and it was complicated to properly customize any of them to work with a given pattern. Plus, they were only designed to simulate lookbehind in a regex-based replacement.

To make it much easier to use lookbehind, I recently posted a collection of short functions on GitHub. They use XRegExp v2, so you should check that out, too.

Here's the code:

// Simulating infinite-length leading lookbehind in JavaScript. Uses XRegExp.
// Captures within lookbehind are not included in match results. Lazy
// repetition in lookbehind may lead to unexpected results.

(function (XRegExp) {

    function prepareLb(lb) {
        // Allow mode modifier before lookbehind
        var parts = /^((?:\(\?[\w$]+\))?)\(\?<([=!])([\s\S]*)\)$/.exec(lb);
        return {
            // $(?!\s) allows use of (?m) in lookbehind
            lb: XRegExp(parts ? parts[1] + "(?:" + parts[3] + ")$(?!\\s)" : lb),
            // Positive or negative lookbehind. Use positive if no lookbehind group
            type: parts ? parts[2] === "=" : !parts

    XRegExp.execLb = function (str, lb, regex) {
        var pos = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === {
                return match;
            pos = match.index + 1;
        return null;

    XRegExp.testLb = function (str, lb, regex) {
        return !!XRegExp.execLb(str, lb, regex);

    XRegExp.searchLb = function (str, lb, regex) {
        var match = XRegExp.execLb(str, lb, regex);
        return match ? match.index : -1;

    XRegExp.matchAllLb = function (str, lb, regex) {
        var matches = [], pos = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === {
                pos = match.index + (match[0].length || 1);
            } else {
                pos = match.index + 1;
        return matches;

    XRegExp.replaceLb = function (str, lb, regex, replacement) {
        var output = "", pos = 0, lastEnd = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === {
                // Doesn't work correctly if lookahead in regex looks outside of the match
                output += str.slice(lastEnd, match.index) + XRegExp.replace(match[0], regex, replacement);
                lastEnd = match.index + match[0].length;
                if (! {
                pos = match.index + (match[0].length || 1);
            } else {
                pos = match.index + 1;
        return output + str.slice(lastEnd);


That's less than 0.5 KB after minification and gzipping. It provides a collection of functions that make it simple to emulate leading lookbehind:

  • XRegExp.execLb
  • XRegExp.testLb
  • XRegExp.searchLb
  • XRegExp.matchAllLb
  • XRegExp.replaceLb

Each of these functions takes three arguments: the string to search, the lookbehind pattern as a string (can use XRegExp syntax extensions), and the main regex. XRegExp.replaceLb takes a fourth argument for the replacement value, which can be a string or function.

Usage examples follow:

XRegExp.execLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", XRegExp("(?i)(?<first>c)at"));
// -> ["cat", "c"]
// Result has named backref: result.first -> "c"

XRegExp.execLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> null

XRegExp.testLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i);
// -> true

XRegExp.testLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> false

XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i);
// -> 18

XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i);
// -> 0

XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<=fluffy\\W+)", /cat\w*/i);
// -> ["cats"]

XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<!fluffy\\W+)", /cat\w*/i);
// -> ["Catwoman", "cats"]

XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<=fluffy\\W+)", /cat/ig, "dog");
// -> "Catwoman's fluffy dog is a cat"

XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, "dog");
// -> "dogwoman's fluffy cat is a dog"

XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, function ($0) {
    var first = $0.charAt(0);
    return first === first.toUpperCase() ? "Dog" : "dog";
// -> "Dogwoman's fluffy cat is a dog"

Easy peasy lemon squeezy. 🙂