$re = '/^\.(_)?[a-z]+-[a-z0-9-]+((_{2}|-{2})?[a-z0-9-]+)?(-{2}[a-z0-9-]+)?[a-z0-9]$/m';
$str = '.o-layout
.c-btn--
.o-layout__item
.has-dropDown
.o-layout--fixed
.u-color-primary
.foo_bar
.s-cms-content
.title__sub
.t-light
.c-date-picker__calendar
.heading
.o-matrix__item--full
._c-footer__copyright';
preg_match_all($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