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Asked: 2026-05-26T13:53:10+00:00

I would like to know how can I construct a regex to know if

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I would like to know how can I construct a regex to know if a number in base 2 (binary) is multiple of 3. I had read in this thread Check if a number is divisible by 3 but they dont do it with a regex, and the graph someone drew is wrong(because it doesn’t accept even numbers). I have tried with: ((1+)(0*)(1+))(0) but it doesn’t works for some values. Hope you can help me.

UPDATE:
Ok, thanks all for your help, now I know how to draw the NFA, here I left the graph and the regular expresion:

In the graph, the states are the number in base 10 mod 3.

For example: to go to state 1 you have to have 1, then you can add 1 or 0, if you add 1, you would have 11(3 in base 10), and this number mod 3 is 0 then you draw the arc to the state 0.

Whiteboard version

((0*)((11)*)((1((00) *)1) *)(101 *(0|((00) *1 *) *0)1) *(1(000)+1*01)*) *

And the other regex works, but this is shorter.

Thanks a lot 🙂

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1 Answer

  1. Editorial Team

    If I may plug my solution for this code golf question! It’s a piece of JavaScript that generates regexes (probably inefficiently, but does the job) for divisibility for each base.

    This is what it generates for divisibility by 3 in base 2:

    /^((((0+)?1)(10*1)*0)(0(10*1)*0|1)*(0(10*1)*(1(0+)?))|(((0+)?1)(10*1)*(1(0+)?)|(0(0+)?)))$/

    Edit: comparing to Asmor’s, probably very inefficient 🙂

    Edit 2: Also, this is a duplicate of this question.

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