gistfile1.php

<?php
$input = "root(27,3)";
$number = '(?:-?\d+(?:\.\d+)?)';
while (!preg_match('/^'.$number.'$/', $input)) {
	$input = preg_replace_callback('/\(\s*('.$number.')\s*\)/', function ($matches) {
		return $matches[1];
	}, $input);
	
	$input = preg_replace_callback('/('.$number.')\s*\^\s*('.$number.')/', function ($matches) {
		return pow($matches[1], $matches[2]);
	}, $input);
	
	$input = preg_replace_callback('/root\(\s*('.$number.')\s*,\s*('.$number.')\s*\)/', function ($matches) {
		return pow($matches[1], 1 / $matches[2]);
	}, $input);
	
	$input = preg_replace_callback('/sqroot\(\s*('.$number.')\s*\)/', function ($matches) {
		return pow($matches[1], 0.5);
	}, $input);
	
	$input = preg_replace_callback('/('.$number.')\s*\*\s*('.$number.')/', function ($matches) {
		return $matches[1] * $matches[2];
	}, $input);
	
	$input = preg_replace_callback('/('.$number.')\s*\/\s*('.$number.')/', function ($matches) {
		return $matches[1] / $matches[2];
	}, $input);
	
	$input = preg_replace_callback('/('.$number.')\s*\+\s*('.$number.')/', function ($matches) {
		return $matches[1] + $matches[2];
	}, $input);
	
	$input = preg_replace_callback('/('.$number.')\s*-\s*('.$number.')/', function ($matches) {
		return $matches[1] - $matches[2];
	}, $input);
	
	echo $input."\n";
}