Re: print real newlines in place of literal ones

Treat the Hello\nWorld\n! as a piece of Perl code to be evaluated:

#!/usr/bin/perl

use strict;
use warnings;

my $line = 'TEXT=Hello\nWorld!\n'; # No interpolation
$line =~ s/^TEXT=//g;
print "$line\n";
my $interpolated = eval "qq{$line}";
print $interpolated;
[download]

Output:

Hello\nWorld!\n
Hello
World!
[download]

Arjen

Comment on Re: print real newlines in place of literal ones Select or Download Code

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Re: Re: print real newlines in place of literal ones by antirice (Priest) on Jun 05, 2003 at 23:08 UTC
Uh... Just a note that this can be a little dangerous. In the worst case, the eval will mess up. But if someone just wants to mess with you, they can do a very nice job of it. Observe: my $line = 'TEXT=};print `ps`;print "Your machine is h4x0r3d!!$/";qq{H +ello\nWorld!\n'; .. OUTPUT: PID TT STAT TIME COMMAND 5808 p0 S 0:00.02 _su (csh) 5816 p0 S+ 0:00.01 perl mylittleprogram.pl 5817 p0 R+ 0:00.00 ps 149 v0 IWs+ 0:00.00 /usr/libexec/getty Pc ttyv0 Your machine is h4x0red!! Hello World # [download] I'm just showing that this can be a little dangerous but if you're the only one who is writing to the config file, you'll need to make certain you never use } or you'll most likely get $interpolated = undef. Of course, a prankster could put some code in the config file as I just showed and have it do a lot of nifty things and you still get the output you expected. So it could be some time before you'd realize anything is wrong at all. Of course, what are the chances of that? ;-) antirice The first rule of Perl club is - use Perl The ith rule of Perl club is - follow rule i - 1 for i > 1	[reply] [d/l]
Re: Re: Re: print real newlines in place of literal ones by Aragorn (Curate) on Jun 06, 2003 at 07:33 UTC
I know ;-) But the previous posts basically asserted that without some substitution, it wasn't possible to do this. When using this technique, you'll want to turn on taint checking and thus be forced to inspect the input (the `TEXT=...` in this case). But while doing that, it's probably easier to make the substitution right away, thereby removing the need for the `eval` altogether! Sometimes, using this technique (or the `do EXPR` function) is useful for writing a script which uses a configuration file without using a special module for it. It's just another way of doing something, and can be very convenient sometimes. Arjen	[reply] [d/l] [select]