The better solution is to selectively use eval. Set up a hash that will store variable values. Use a regex to translate 'bare' variables in your expressions to point to this hash. Then eval the expression as necessary:

my $exp = "y=2*x+3";
my %vars = ( x => 1 );

$exp =~ s/([a-z]+)/\$vars{\1}/g;
eval $exp;
print $vars{ y };
[download]

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important

If you're not up to the Parse::RecDescent challenge, you can download the code used in chapter 6 ("Global Matching") of my book. Specifically, from 6.6, "Parsing a language". The code is in examples 6.6 and 6.7. It implements a simple calculator with variable support.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

Another possibility is Math::Expr.

After Compline,
Zaxo

So the solution would be:

#You might want to do something to look for say sin, cos, tan, etc.
unless( $str =~ /[^0-9+-*^/.,_()]/ ){
  eval($str);
}
[download]

--
perl -p -e "s/(?:\w);([st])/'\$1/mg"

Or use the Safe module and mask the operators that can be considered potentially harmful. This has the benefit of allowing the "math expression" to be arbitrarily complex (Perl, really), and if you pre-loaded things like Math::Complex, you could potentially have a very powerful "calculator."

$a="(1/2)*4"; 
print eval($a),"\n";
[download]

--
tune

One issue that I didn't see touched on was execution speed. It appears that you'd want to calculate, for example, (X/2)^4 for quite a few values of X. The fastest way to do this would be to create a subroutine that does the calculation. For example, a closure:

my $sub= eval "sub { my($x)=@_; return ($x/2)**4 }";
die $@   if  $@;
for my $x (  0..100  ) {
    $x /= 10;
    my $y= $sub->( $x );
    plot( $x, $y );
}
[download]

So the trick is deciding what to allow that will be safe.

If you only allow digits and mathish punctuation, then I can't come up with a way that the result would be dangerous:

if(  $equ =~ m#[^-\s\d+*/()]#  ) {
    # refuse to use it
[download]

my $equ= $q->param("equation");
my( $var, $expr )= $equ =~ /^\s*(\w+)\s*=(.*)/
  or  fail("Invalid equation");
my @words= qw( x y z abs atan2 cos exp log sin sqrt );
my %words;
@words{@words}= (" ") x @words;
( my $check= $expr ) =~ s/(\w+)/ $words{$1} ? "" : "x" /ge;
if(  $check =~ m#([^-\s\d+*/()]+)#  ) {
    fail( "Disallowed function/variable ($1)" );
}
@words{qw(x y z)}= qw( $x $y $z );
$expr =~ s/(\w+)/ $words{$1} /ge;
my $sub= eval 'sub { my($x,$y,$z)= @_; return ' . $expr . '}';
fail( "Invalid expression ($expr): $@" )  if  $@;
[download]

You could just punt and pass things off to gnuplot.