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Editorial Team
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Asked: 2026-05-15T04:27:01+00:00

I wanted to detab my source files. (Please, no flame about WHY I wanted

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I wanted to detab my source files. (Please, no flame about WHY I wanted to detab my sources. That’s not the point 🙂 I couldn’t find a utility to do that. Eclipse didn’t do it for me, so I implemented my own.

I couldn’t fit it into a one liner (-e) program.
I came with the following, which did the job just fine.

while( <> )
{
    while( /\t/ ) {
        s/^(([^\t]{4})*)\t/$1    /;
        s/^((([^\t]{4})*)[^\t]{1})\t/$1   /;
        s/^((([^\t]{4})*)[^\t]{2})\t/$1  /;
        s/^((([^\t]{4})*)[^\t]{3})\t/$1 /;
    }
    print;
}

However, it makes me wonder if Perl – the champion language of processing text – is the right tool. The code doesn’t seem very elegant. If I had to detab source that assume tab=8 spaces, the code would look even worse.

Specifically because I can think of a deterministic state machine with only 4 states to do the job.

I have a feeling that a more elegant solution exists. Am I missing a Perl idiom? In the spirit of TIMTOWTDI I’m curious about the other ways to do it.

u.

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1 Answer

  1. Editorial Team

    I remember a detabify script from one of the O’Reilly books, but I can’t seem to find a link now.

    I have had to solve this problem as well, and I settled on this concise solution to detabify a line:

    1 while $line =~ s/\t/" " x ($tablength - ($-[0] % $tablength))/e ;

    In this regular expression $-[0] is the length of the “pre-matched” portion of the line — the number of characters before the tab character.

    As a one-liner:

    perl -pe '1 while s/\t/" "x(4-($-[0]%4))/e' input
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