在Perl中分割4个空格

Question

I have a file which consists of lines in the following format:

field1    field2    field3

Each field is separated by 4 spaces.

I am trying to extract field2 from the above list using Perl.

So, this is what I have done,

#! /usr/bin/perl

($input, $output) = @ARGV;

chomp $output;

open(INPUT,"<",$input);
open(OUTPUT,">>",$output);

while(<INPUT>)
{
chomp $_;
($f1,$f2,$f3)= split("\t",$_,3); // *** This line was modified later.
print OUTPUT $f2."\n";
}

close INPUT;
close OUTPUT;

This gives me a blank file.

I have tried to modify the line indicated above with an asterisk as:

($f1,$f2,$f3)= split("\t",$_);
($f1,$f2,$f3)= split("    ",$_);
($f1,$f2,$f3)= split("\s\s\s\s",$_);

In all these cases, again I get an empty file.

Answer 1

It may be that your open statement(s) have failed. You should never use open without checking if it failed or not, eg

open my $fh, '<', $file or die $!;

I used three argument open with a lexical file handle, as is recommended. See perldoc -f open for more information.

Such a mistake will be revealed if you are using use warnings , giving an error:

readline() on closed filehandle INPUT at ...

However, if you are not using warnings, the error will be silent and deadly. It goes without saying that you should of course always

use warnings;
use strict;

Answer 2

Your code works if the fields are separated by four spaces as you claim.

$ echo 'field1    field2    field3' | perl -nlE'say for split "    ";'
field1
field2
field3

Verify that your file actually four spaces contains using od -c .

---

Btw, you're lying to yourself by using split "..." instead of split /.../ . The first arg of split is a regular expression. It's clearer if you make it look like one.