Actually, you don't. A tab isn't a certain number of spaces, it's a tab. Various text editors/word processors will represent the tab as a number of spaces. Depending on your settings, they may actually save (X) spaces when you hit tab, rather than save the tab.

Generally you want to convert tabs to spaces or spaces to tabs. My attempt to do what I think you mean is to either:

1. Change the configuration on your text editor to set tabs as 4 spaces OR,
2. do a regex to replace \t with "    "

while (<>){
    s/\t/    /g;
}
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while (<>){
    s/    /\t/g;
}
[download]

Read article 41.04 (p. 754-756) in Unix Power Tools.

In short:

• a tab is just one character, the terminal (or printer, or editor, or ... ) does the conversion to spaces
• the standard Unix utilities "expand" and "unexpand" can be used to convert between tabs and spaces - they are available for Win32 too in the Cygwin package
• for more detail, read the man pages (or the source :-) ... ) of "expand" and "unexpand"

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com

   |   |   |   |   |   |   |   |   |   |
1234567890123456789012345678901234567890
this is\ta test
this is    a test
this was\ta test
this was   a test
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Simple substitution of \t for \s{4} will not work. You must know how many spaces are necessary to reach the next stop.

Text::Tabs will correctly expand and contract tabs for you.

use vars qw($spaces);
my $spaces = " " x shift;
while (<>)
{
    s/\t/$spaces/g; # Tabs to spaces.
                    # Perform substitution the other way 
                    # for spaces to tabs.
    print;
}
#
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There's a program somewhere in textutils, in the GNU toolkit, that will do this too.

Update: Changed according to MeowChow's suggestion.

You left out a few things. I believe you mean:

perl -pe 's/\t/q( )x$n/ge' ...
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   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

A tab is a single character; the only real issue here is how they get displayed by whatever app you're using. You could write a filter that translates \t to four spaces, I guess, but that would be a *translation* and would assuredly *not* preserve the actual character count of the document.