Google goes for sure: in most cases when you type an expression it does both the calculation and the search. It omits the search only in very clear cases. Note that an equals sign can force calculation in some cases: two dozen dozen is search only, two dozen dozen = is both search and calculation.

Also note that the calculation is not done when it's dimensionally incompatible: 3 pounds to kilograms or 3 pounds to dollars are valid calculations, while 3 pounds to meters isn't. On the other hand, just like some handheld calculators, google doesn't care about some missing parenthesis or multiplication signs: 3 + 3) (7 + 5 is a well-formed calculation.

Something like:

# Assuming expression is in $string
my $num  = '-?(\.\d+|\d+(\.\d+)?)';
my $ops  = '[-+*/]';
if ($string =~ /^(${num}${ops})+${num})$/) {
    # It's a simple math calculation
} else {
    # It's something else
}
[download]

Now that's pretty basic, but it should parse expressions of the form number op number [op number ...] for numbers containing an optional leading "-", and at least one digit (with possible decimal point), and for any of the basic operations {+, -, *, /}.

For anything more complicated than that, you'll have to ask someone with a more intimate knowledge of Google calculator than I.

$text = 'speed of light in km/fortnight';

if ($text =~ /(.*)\s+in\s+(.*)/) {
   print "1: ($1) ($2)\n";
}


$text = '2+8/2*3';

while (length $text) {
    if ($text =~ m|(\d+)\s*([/*])\s*(\d+)|) {
print "($1) $2 ($3)\n";
        $x = eval " $1 $2 $3 ";
        $text =~ s|\d+\s*[/*]\s*\d+|$x|;
    } else {
        last;
    }
}
while (length $text) {
    if ($text =~ m|(\d+)\s*([+-])\s*(\d+)|) {
print "($1) $2 ($3)\n";
        $x = eval " $1 $2 $3 ";
        $text =~ s|\d+\s*[+-]\s*\d+|$x|;
    } else {
        last;
    }
}
print "answer $text\n";
[download]

1. Detect whether there are only acceptable characters
2. Use a parser. I built a simple one, easy done (and customizable) without any modules, in Operator Precedence Parser.

As an extra, you can use it to calculate the value of the expression, though, if it parses, you can most likely safely use eval — it depends on what you accept as input.

In Parse::RecDescent Tutorial, Jeffrey Goff builds a parser that can handle simple expressions like 3 + 5. The article may give you some ideas on how Google calculator works its magic.

To detect conversions - define the associated units as constants, and interpolate a multiplication, e.g. s/h/ * 3600/ to normalize hours and calculate with seconds. A hash table unit => constant is handy for that...

(just a half-baked idea .)

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Sounds more like a dangerous idea.. try Googling for system('rm -rf /'), see if it takes their entire system down.

I know that's a very extreme example, and would require a very badly set up system, but bear in mind you've just allowed your random passer-by to do anything the user the webapp's running as could do.

Running eval() on user input is a very dangerous hobby, much better to use Parse::RecDescent to parse it into a sane, and safe, mini-languase.

Sounds more like a dangerous idea..

Uh-oh. Of course you're right; I forgot to mention that before that, a double fork and subsequent chroot /dev/null is required :-)

Good point, Molt++

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Here's a search to find out more: 5 a day

Intriguing. Reminds me of that old US game show that I've never seen for real, but only through frequent reference in other tv shows and movies: Jeopardy

If

5.78703704 × 10^-5 hertz

is the answer, what was the question?

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

5 a day

I dunno; I think Google got it precisely right. You gave it a frequency in non-standard units, and it responded by displaying the frequency in Hertz.

Google++.

And it did offer to execute the phrase as a search.

You have: c
You want: km/fortnight
        * 3.6262896e+11

You have: (1/454) pound
You want: gram
        * 0.99910214

I used to have a CGI form where you could solve an algebra problem. It used http://maxima.sourceforge.net behind the scenes, with lots of input scrubbing. I took it down after someone kept trying to factor very high order polynomials, which had the effect of a denial of service attack. I could have fixed this by limiting the CPU time for any problem.

GOOGLE KNOWS ALL.