Re: [PATCH] perlfaq4: How can I expand variables in text strings?

[email protected] (Robert Spier) writes:

> What is tainting?  (I know.  I'm being rhetorical.) That seems outside
> the scope of the question.

OK here's an alternative, simpler version, without any mention of
tainting.

> > +    $text =~ s/(\$\w+)/$1/eeg; # needed /ee, not /e
> > -    die if $@;			# needed /ee, not /e
> 
> I'm not sure why you removed the error checking.

Now I remember why I did that.  The error checking given in the
current version of the FAQ is, in fact, bogus.

s///eeg does an eval() in a loop, checking $@ outside will only see
the value from the last iteration.

If you want error checking you need to do it inside the loop:

   $text =~ s/(\$\w+)/my $v = eval $1; die if $@; $v/eg;

I think that is probably too complex and confusing for the FAQ.
Better just to leave it out.

Here's the revised FAQ entry:
 
       How can I expand/interpolate variables in text strings?

       To process a string through Perl's interpolation engine
       simply:

           $text = 'this has a $foo in it...\n  ...and a $bar';
           # Assume $text does not contain "\nEND\n"
           chop ( $text = eval "<<END\n$text\nEND\n" );
           die if $@;

       For an explanation of how $text could execute arbitrary
       Perl see ``How do I expand function calls in a string?''
       in this section of the FAQ.  If you do not trust the
       source of $text not to try evil things you can limit (and
       launder) the parts of $text that are passed to eval():

           $text =~ s/(\$\w+)/$1/eeg; # needed /ee, not /e

       This still gives unrestricted access to scalar variables.
       It is often better to use a hash:

           %user_defs = (
               foo  => 23,
               bar  => 19,
           );
           $text =~ s/\$(\w+)/$user_defs{$1}/g;

       For other variations on the theme of text templates see
       the sprintf() function and numerous modules on CPAN.

The patch against perl-5.8.1-RC4 for this version can be found at:

http://www.wcl.bham.ac.uk/pub/bam/patches/perl/perlfaq4-scalar-interpolate-take-5.diff