using re 'eval' for varibable math

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I just discovered the pragma 're' and I'm inetersted in using it. If I had the string:

my $question="67.35 >= 50 && 67.35 < 80";
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How would I complete the ev routine?

#!C:/perl/bin/perl.exe

use warnings;
use strict;
use re 'eval';

my $question="67.35 >= 50 && 67.35 < 80";

my $ans=ev($question);
print "ans=$ans\n";

sub ev
{    ... ?
}
[download]

Or is there a better way?

Comment on using re 'eval' for varibable math Select or Download Code

Replies are listed 'Best First'.
Re: using re 'eval' for varibable math by ikegami (Patriarch) on Apr 19, 2005 at 14:59 UTC
That's not a regexp, so the `re` pragma won't help here. However, the built-in `eval` function will: `#!C:/perl/bin/perl.exe use warnings; use strict; my $question = "67.35 >= 50 && 67.35 < 80"; my $ans = eval $question; die("Compile error: $@\n") if $@; print("ans = ", ($ans ? 'true' : 'false'), "\n");` [download] Beware including user-supplied data in an eval. Users are sometimes malicious or supply unexpected data.	[reply] [d/l] [select]
Re^2: using re 'eval' for varibable math by Anonymous Monk on Apr 19, 2005 at 15:03 UTC
Much easier....That works great. Thanks!	[reply]
Re: using re 'eval' for varibable math by tall_man (Parson) on Apr 19, 2005 at 20:53 UTC
The Math::Pari module can evaluate mathematical expressions with a more restricted syntax than perl's -- but watch out for "system()". Here's an example. `use strict; use Math::Pari qw(PARI); my $question="67.35 >= 50 && 67.35 < 80"; # Pari doesn't like white space in expressions. $question =~ s/\s+//g; my $out = PARI $question; print $out,"\n";` [download]	[reply] [d/l]
Re: using re 'eval' for varibable math by salva (Canon) on Apr 19, 2005 at 15:55 UTC
as ikegami toll you, `eval $expr` is the easiest way to evaluate arithmetic expresions but it allows for more things that just math, so it can be very dangerous. You can try checking the expression first with a regular expression but you have to be very carefull, and finding a secure filter for more than simple math expressions becomes difficult. Other alternative is to write a parser and an evaluator in perl. The parser converts the string to a tree (or stack or other conveniant representation) and after that, the evaluator evaluates it (look in CPAN for Parser::RecDescent for an easy way to write parsers).	[reply] [d/l]