$re = '/\w*(\.([a-zA-Z0-9]+[-_]+)?careerfy(?![^\W_])([-_]+[a-zA-Z0-9]+)?)\w*/m';
$str = '.crfy-careerfy { display: none}
.careerfy { font-size: 12; font-family: "arial"}
.crfy-careerfyds-add-space {background-color: red}
.crfy-careerfy_add-space {background-color: red}';
preg_match_all($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