PHP - Parse mathematical equations inside strings - php

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.
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.
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;
}
?>

Answer 2

preg_match_all('/\[(.*?)\]/', $string, $out);
foreach ($out[1] as $k => $v)
{
eval("\$result = $v;");
$string = str_replace($out[0][$k], $result, $string);
}
This code is highly dangerous if the strings are user inputs because it allows any arbitrary code to be executed

Answer 3

The eval approach updated from PHP doc examples.
<?php
function calc($equation)
{
// Remove whitespaces
$equation = preg_replace('/\s+/', '', $equation);
echo "$equation\n";
$number = '((?:0|[1-9]\d*)(?:\.\d*)?(?:[eE][+\-]?\d+)?|pi|π)'; // What is a number
$functions = '(?:sinh?|cosh?|tanh?|acosh?|asinh?|atanh?|exp|log(10)?|deg2rad|rad2deg|sqrt|pow|abs|intval|ceil|floor|round|(mt_)?rand|gmp_fact)'; // Allowed PHP functions
$operators = '[\/*\^\+-,]'; // Allowed math operators
$regexp = '/^([+-]?('.$number.'|'.$functions.'\s*\((?1)+\)|\((?1)+\))(?:'.$operators.'(?1))?)+$/'; // Final regexp, heavily using recursive patterns
if (preg_match($regexp, $equation))
{
$equation = preg_replace('!pi|π!', 'pi()', $equation); // Replace pi with pi function
echo "$equation\n";
eval('$result = '.$equation.';');
}
else
{
$result = false;
}
return $result;
}
?>

Answer 4

Sounds, like your homework....but whatever.
You need to use string manipulation php has a lot of built in functions so your in luck. Check out the explode() function for sure and str_split().
Here is a full list of functions specifically related to strings: http://www.w3schools.com/php/php_ref_string.asp
Good Luck.