regular expressions

13 Articles

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Visualize Regular Expressions With Marbles

October 20, 2026 by 16 Comments

If you are one of those people who visualize things physically better than in the abstract, then Regex Marble Run by [bottomless] might be for you. You enter a test expression and a string, and the site generates a virtual marble machine that lets you see the regular expression pick apart the string in a physical way.

Click this link, and you’ll see the regular expression “^[hH]ack.[dD][^0-9]*$” which, if you aren’t a regular expression user, might seem a bit cryptic. However, it is fairly simple if you take it bit-by-bit.

The caret character tells it to match the start of the string. So “^dog” will match “dog” and “dog food” but won’t match “food for a dog.” The following bit “[hH]” will match either “h” or “H”. Then there is “ack” which is just what you think it is. So a string starting with “hack” or “Hack” might match (depending on the rest of the expression.

The period means match any single character. Then there’s another “[]” but this time with “dD” to match any kind of D. The next “[]” character class is a range from 0-9, but since there is a caret, the match is negated. So “[^0-9]” means match anything except a digit from 0 to 9. The “*” means we can have zero or more of these. So “HackxD” matches, but so does “HackxDXYZabc” for now. Trick question: Does “HackxDXYZabc123” match? So far, yes, but it only matches to the lowercase “c” character.

The final character, “$” means end of string. So now “HackxDXYZabc123” does not match because we are still in the middle of the string. However, “HackxDXYZabc” will match since the previous “*” “ate” the rest of the string.

Head spinning? The site will let you see each letter of a test string as a marble that falls through the different parts of the regular expression. Balls that pass wind up in the left-hand box, while failures are in the right-hand box. Will it help you? We don’t know, but it is fun to watch.

Want a refresher on regular expressions? Or perhaps you want some practice?

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DOOM Using Regular Expressions

July 28, 2026 by 20 Comments

Regular expressions (regexes) are an amazingly powerful way to perform operations on collections of e.g. text. Regexss can also be considered to be a programming language, even a Turing complete one. Ergo it’s perfectly acceptable to thus design a way to use regexes to run and play a game of DOOM, as [Artem Lytkin] recently did.

The GitHub project page can be found here, containing the Python-based code that allows the demonstration to run, as well as the other components, including the C runtime and the 96.6 MB text string that defines a CPU’s registers, RAM, a framebuffer, the DOOM engine compiled to this custom CPU’s instruction set and the WAD file for the game itself.

The C-based driver applies the fixed, ordered list of find-and-replace rules to this string, which after more than ten-thousand of such substitutions later results in a single frame of the game. At about 80,000 substitutions per second on the given test system, that gets you to a sort-of playable framerate, even.

Naturally, the practical value of playing DOOM like this is pretty low, but as a demonstration of why regexes are awesome it’s hard to beat.

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Regular (Expression) Chess

January 7, 2025 by 17 Comments

[Nicholas Carlini] found some extra time on his hands over the holiday, so he decide to do something with “entirely no purpose.” The result: 84,688 regular expressions that can play chess using a 2-ply minmax strategy. No kidding. We think we can do some heavy-duty regular expressions, but this is a whole other level.

As you might expect, the code to play is extremely simple as it just runs the board through series of regular expressions that implement the game logic. Of course, that doesn’t count the thousands of strings containing the regular expressions.

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Regular Expressions Finally Come To Microsoft Excel

May 24, 2024 by 29 Comments

There are two types of people in the world: those who have no idea what a regular expression is, and those who not only know what they are but can compose them on the fly and tend to use them in situations where they’re clearly not called for. And it’s that latter camp, of which we consider ourself a proud member, that is rejoicing with the announcement that Microsoft is adding regular expression support to Excel.

Or perhaps not rejoicing so much as wondering what took so long. Yes, regular expressions have been part of VBA for a while now, but the new functions allow you to use regexes right in the spreadsheet grid. There are plenty of caveats, of course. The big one is that this is still in beta at this time, so you have to do some gymnastics to enable it, if you’re even allowed to in the first place. Second, support appears limited to three functions at the moment: REGEXTEST, which provides a logical test of pattern matching; REGEXEXTRACT, which returns a substring that matches a pattern; and REGEXREPLACE, which substitutes a string for a pattern. The video below walks through how to use these functions within spreadsheets.

What’s also unclear now is what flavor of regular expressions is supported. There are a bewildering number of entities in the regex bestiary — character classes, positional indicators, quantifiers, subexpressions, lazy and greedy matches, and a range of grouping constructs that perplex even regex pros. One hopes these new functions will support one of the existing regex standards, but Microsoft is famous for “extending and enhancing.” Then again, regex support has been in the .NET Framework for years and is pretty close to the Perl standard, so our guess is that it’ll be close to that.

If you fall into the “What’s a regex?” camp but want to change that, why not get your grep on?

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Linux Fu: Globs Vs Regexp

October 7, 2021 by 17 Comments

I once asked a software developer at work how many times we called fork() in our code. I’ll admit, it was a very large project, but I expected the answer to be — at most — two digits. The developer came back and read off some number from a piece of paper that was in the millions. I told them there was no way we had millions of calls to fork() and, of course, we didn’t. The problem was the developer wasn’t clear on the difference between a regular expression and a glob.

Tools like grep use regular expressions to create search patterns. I might write [Hh]ack ?a ?[Dd]ay as a regular expression to match things like “HackaDay” and “Hack a day” and, even, “Hackaday” using a tool like grep, awk, or many programming languages.

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13,000 Regular Expressions Make An Editor’s Life Easier

February 1, 2021 by 41 Comments

Being an editor is a job that seems deceptively easy until you are hauled over the coals for letting a textual howler go to print (or website). Most publications have style guides to ensure that their individual voice is preserved, but even the most eagle-eyed will sometimes slip up in their application. At the Guardian newspaper in the UK they have been struggling with this against an ever-evolving style guide that must adapt to fast-moving world events, to the extent that they had a set of regular expressions to deal with commonly-occurring problems. A lot of regular expressions, in fact around 13,000 of them.

Clearly some form of management was required, and  a team of developers set about taming this monster. The result is Typerighter, their server-side document-checker, which can be found in a GitHub repository. Surprisingly for rule management they started with a Google Sheet, a choice which proved unexpectedly robust when working with such a long list even though they later replaced it. The back end doing the job of text matching was written in Scala, and for the front end a plugin was created for their Prosemirror text editor.

For a publication of course this is extremely interesting, but where’s the interest for hackers? The answer lies in any text-processing engine that uses a lot of regular expressions; those of you who have dabbled in this space will know how unwieldy this work can become. Any user of computational linguistic techniques in the pursuit of language processing could probably find much of interest here.

If you’re a bit hazy on regular expressions, how about the episode on them from our long-running Linux-fu series?

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DIY Regular Expressions

October 29, 2020 by 12 Comments

In the Star Wars universe, not everyone uses a lightsaber, and those who do wield them had to build them themselves. There’s something to be said about that strategy. Building a car or a radio is a great way to learn how those things work. That’s what [Low Level JavaScript] points out about regular expressions. Sure, a lot of people think they are scary. So why not write your own regular expression parser and engine? Get that under your belt and you’ll probably never fear another regular expression.

Of course, most of us probably won’t do it ourselves, but you can still watch the process in the video below. The code is surprisingly short, but don’t expect all the bells and whistles you might find in Python or even Perl.

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