PHP - Parse mathematical equations inside strings

Question

I'm struggling to find the best way to do this. Basically I am provided strings that are like this with the task of printing out the string with the math parsed.

Jack has a [0.8*100]% chance of passing the test. Katie has a [(0.25 + 0.1)*100]% chance.

The mathematical equations are always encapsulated by square brackets. Why I'm dealing with strings like this is a long story, but I'd really appreciate the help!

Answer 1

There are plenty of math evaluation libraries for PHP. A quick web search turns up this one.

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Writing your own parser is also an option, and if it's just basic arithmetic it shouldn't be too difficult. With the resources out there, I'd stay away from this.

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You could take a simpler approach and use eval. Be careful to sanitize your input first. On the eval docs's page, there are comments with code to do that. Here's one example:

Disclaimer: I know eval is just a misspelling of evil, and it's a horrible horrible thing, and all that. If used right, it has uses, though.

<?php

$test = '2+3*pi';

// Remove whitespaces
$test = preg_replace('/\s+/', '', $test);

$number = '(?:\d+(?:[,.]\d+)?|pi|π)'; // What is a number
$functions = '(?:sinh?|cosh?|tanh?|abs|acosh?|asinh?|atanh?|exp|log10|deg2rad|rad2deg|sqrt|ceil|floor|round)'; // Allowed PHP functions
$operators = '[+\/*\^%-]'; // Allowed math operators
$regexp = '/^(('.$number.'|'.$functions.'\s*\((?1)+\)|\((?1)+\))(?:'.$operators.'(?2))?)+$/'; // Final regexp, heavily using recursive patterns

if (preg_match($regexp, $q))
{
    $test = preg_replace('!pi|π!', 'pi()', $test); // Replace pi with pi function
    eval('$result = '.$test.';');
}
else
{
    $result = false;
}

?>