$re = '/(?<!-)\b(?![x|\-|\+]\s?)(may|may15)(?!\s[x|\-|\+])\b(?!-)(\s|\,)/i';
$str = 'TRADED; Mar15 x May15, 5 MW Base 121,50 TL';
preg_match_all($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