- preg_match: documentation ( source)
- preg_replace: documentation ( source)
<?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, $test))
{
$test = preg_replace('!pi|π!', 'pi()', $test); // Replace pi with pi function
eval('$result = '.$test.';');
}
else
{
$result = false;
}