$re = '/(?:[-+*\/0-9]+\s*)+/m';
$str = 'простая арифметика.
простое выражение 22+8
выражения с случайными пробелами 11* 8 или 11 *8 или 11 * 8
выражение посложнее -99*8+3
';
preg_match_all($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