$re = '~^((-?\d+(?:\.\d+)?)(?:[*+/^eE-]| (?:to the power of|plus|add|minus|subtract|divided?|times|multipl(?:y|ied))(?: by)?)(?: )?)(?1)*(?2)$~m';
$str = '2*2
2+2
2/2
2-2
2^2-2
2^2
2e10^4
2^2^2^2+4
2+2+2+2
2 times 2
2 multiplied by 2
2 multiply 2
2 divide 2
2 divided by 2
2 add 2
2 plus 2
2 minus 2
2 subtract 2
2 to the power of 2 divided by 10
2 multiplied by 10 times 3
2.09187 times 9
-3 times 9
2+-6
2 multiplied by';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php