Regex Chess: A 2-ply minimax chess engine in 84,688 regular expressions

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ssurprisetalk 6 days ago 49 commentsRead Article on nicholas.carlini.com

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Discussion (49 Comments)Read Original on HackerNews

LukaD•1 day ago

This is delightfully insane! I don't think I would say it doesn't play _entirely_ terrible though ;) It's playing really bad, but it could be worse and it's already super impressive that it can even generate legal moves.

Kaliboy•1 day ago

This is amazing. I'm at loss for words.

During my CS years I remember being fascinated by NFA's, as opposed to boring single universe DFA's.

For some reason I internalized that I would never see something like an NFA implemented beyond text books.

Then came Carlini.

bigdict•1 day ago

But... they are equivalent?

Kaliboy•1 day ago

Yeah I know, but I thought I was doing purely theoretical excercises.

And we always changed the regex NFA to an equivalent DFA and that was the implementation.

So somehow I managed to internalize the idea that an NFA is purely theoretical and can't be built.

xpon•1 day ago

Modulo an exponential blowup! That’s like saying P is equivalent to NP.

tgv•1 day ago

Depends on what you mean by that. You can convert every NFA into a DFA. That's a NP complete (IIRC), but running the DFA is O(n). Running the NFA without converting it is also NP complete. One isn't better than the other, but the costs vary for different expressions and usages.

froh•1 day ago

The blow up is exponential for carefully crafted academical regular expressions.

im practice is a good idea to build a DFA from your regex, up front (re2) or lazily (ripgrep)

pkal•1 day ago

No, because you can compute the optimal automaton (as in least number of states) that recognizes the same language: https://en.wikipedia.org/wiki/DFA_minimization

zelphirkalt•1 day ago

It would be different, if somehow all those 84688 regexes were coded by hand. Then it would be a piece of art.

It would be different, if the number of regexes was maybe below 300, and it still plays acceptably. The sheer number of regexes kind of defeats the purpose.

At that code size, a much better engine can be written, or other kind of code for an engine be generated. Regexes themselves are not really something we should strive to use more either. Maybe its intentional badness kind of makes it art?

colonCapitalDee•1 day ago

What a depressing way to view the world. Sorry this great project wasn't to your taste, but there's no reason to be a dick about it.

matja•1 day ago

I was also thinking along the same lines. Interesting, but I'm not sure in which aspect it is an achievement, considering the loop isn't a regex.

Meanwhile, 1K ZX Chess takes fewer bytes of memory than the first four paragraphs from the post.

senfiaj•1 day ago

> Maybe its intentional badness kind of makes it art?

I guess it's the whole point of such type of blog posts. Similarly, some people write complicated interactive web pages without using JS, like this https://benjaminaster.com/css-minecraft/. But if you look at the HTML / CSS code size, it's usually huge, but still requires creativity to do that because of constraints. Obviously, it's not something practical or even optimal.

latexr•1 day ago

> Similarly, some people write complicated interactive web pages without using JS (…) Obviously, it's not something practical or even optimal.

There are people who navigate the web with JavaScript turned off, so those experiments do have practical applications.

There are entire projects around not using JavaScript.

https://theosoti.com/you-dont-need-js/

https://github.com/you-dont-need/You-Dont-Need-JavaScript

senfiaj•1 day ago

> There are people who navigate the web with JavaScript turned off, so those experiments do have practical applications.

This is practical (and necessary) for relatively basic stuff, such as text content, navigation, basic form / input validation, and things like that. But when people write more complicated things (requiring state management, logical branches, etc), like games, 3d programs, etc, it's much more challenging (also can be sub-optimal) and requires more creativity. I mean they are more of a demo art rather than some strong necessity.

LatencyKills•1 day ago

This is a quintessential, crazy idea that used to be adored on HN. The author, obviously, didn't intend this to be a serious engine.

I wish more submissions began with, “This might be a bit wild, but I wanted to see if it could actually work.”

Timwi•1 day ago

> “This might be a bit wild, but I wanted to see if it could actually work.”

That is how esoteric programming languages start.

BretonForearm•1 day ago

Out of curiosity, why wouldn't it work?

LatencyKills•1 day ago

Oh, I didn't mean that this specific project wouldn't work. I just wish HN were a little friendlier towards projects that are primarily thought experiments.

Some of the best things I've ever created started from, "I wonder what would happen if I tried this crazy approach..."

evilsnoopi3•1 day ago

The technical write up is worth perusing but I played a game before reading and accidentally found a winning strategy immediately. I'm not sure if this is a result of the 2-ply nature of the engine or if the mentioned deficiencies account for this but the computer did not act to prevent checkmate in 1 (without any intervening check); the game I played was (in algebraic notation): 1. e4 e5 2. kf3 kf6 3. kxe5 kxe4 4. d4 kxf2 5. Kxf2 a5 6. Qf3 b5?? 7. Qxf7 1-0

EvgeniyZh•1 day ago

Yep, the scoring function is just piece value difference, so it can only detect checkmate in 0 (i.e., when king capture is available).

zelphirkalt•1 day ago

Nitpick: In chess usually "N" is used to mean "knight", because "K" is already taken by "King".

FergusArgyll•1 day ago

Hey! I had a very similar game

Rendello•1 day ago

This reminded me of Tom7's video where he made a bunch of ridiculous engines and pitted them against each other (and against "diluted" versions of Stockfish):

https://www.youtube.com/watch?v=DpXy041BIlA

https://tom7.org/chess/

strenholme•1 day ago

For people who are interested, here is the solution. In standard PGN, the solution is:

1. e4 e5 2. Nf3 Nf6 3. Nxe5 Nxe4 4. Qe2 Nxd2 5. Nc6+ Ne4 6. Nxd8 Kxd8 7. Qxe4 a6 8. Bg5+ Be7 9. Qxe7#

In the Stockfish notation this engine uses, White’s moves are:

1. e2e4 2. g1f3 3. f3e5 4. d1e2 5. e5c6 6. c6d8 7. e2e4 8. c1g5 9. e4e7

Here is a Lichess analysis of this game:

https://lichess.org/WnMF3LpX

(In terms of Regexes, Javascript has a very rich Turing complete Regex library; it’s an open question whether Lua 5.1’s regexes are Turing complete, but they are good enough for the text processing I do)

zzazzdsa•1 day ago

I won with 1. e4 e5 2. Qh5 a6 3.Bc4 a5 4. Qxf7#. I wonder if you could implement a stronger engine in regex (stockfish classic at O(1) nodes is plenty strong already)

wavemode•1 day ago

I won faster than that:

1.d4 d5 2.c4 dxc4 3.Nc3 Qxd4 4.Qxd4 a6 5.Bf4 a5 6.Bxc7 a4 7.Qd8#

VladVladikoff•1 day ago

This is like a fever dream.

userbinator•1 day ago

Upon reading the title, this is one of those "I know that's possible, but I'd never bother to implement it" things, although this particular implementation isn't exactly what I had in mind.

deviation•1 day ago

Not sure it's completely accurate. I played a standard queen's gambit accepted, took black's queen which it immediately blundered, then tried to move my queen from c5 -> e5 and the game ended immediately showing:

  *Illegal Move*
  You Lose.
  Game over.

A little disappointed, since it's of course a valid move.

Someone•1 day ago

“then tried to move my queen from c5 -> e5”

Are you sure you typed “c5e5”?

It’s very picky about how you specify a move. “e2e4” is fine as a first move, for example, but auto-capitalized “E2e4” is losing immediately. Quite weird, given that there are guardrails against “e2-e4” and “E2-E4” (an alert pops up telling you how to write moves)

benchloftbrunch•1 day ago

Yeesh, one illegal move attempt means you just lose? That's harsh...

asplake•1 day ago

And now you have 84,689 problems

explodes•1 day ago

2025

dtj1123•1 day ago

Brilliant. The Chinese room thought experiment as a chess engine.

devanshp•1 day ago

This is absurd. I did not realize you could do nearly this much computation in regex.

tgv•1 day ago

It's not just regex. The regular expressions are used to select and perform an action. There's a loop around it with controls the stack. That has more power than the regex.

karlgkk•1 day ago

It’s turing complete so you could compile almost any language to regex. You might have to build a vm for some languages, also in regex. The point is, it’s regex all the way down.

Patryk27•1 day ago

Regular expressions are not Turing-complete.

0xffany•1 day ago

True in the CS Theory space, but most modern regex engines implement a few niceties which make their "regex" turing complete. https://blog.poisson.chat/posts/2024-06-18-turing-regex.html

benchloftbrunch•1 day ago

Javascript/PCRE/etc regexes have additional features (like backreferences) that give them strictly more computational power than a regular DFA/NFA. (Still not Turing complete though without external control flow to support arbitrary iteration/recursion, like is done here)

casey2•about 8 hours ago

This is an odd comment because it's a famously (imo) over known fact due to cs textbooks and how academia organizes knowledge, optimizing for pushing papers over genuine discovery.

neuroelectron•1 day ago

Previously posted: https://news.ycombinator.com/item?id=42619652

carlsborg•1 day ago

"Memory plus search is all you need"

casey2•1 day ago

Alternate title:

Compiling Python to a Branch-Free SIMD Virtual Machine via Extended Regular Expression String Rewriting