I can't find a correct regular expression to extract classes names. It is my code:
preg_match_all('@\.(.*!\.){display:none}@', '.UlEa{display:none}.RNLW{display:inline}.jc0k{display:none}.Lyhf{display:inline}', $matches);
The $matches is empty. How should look correct regular expression?
Note that your regex matches a literal ., then captures a sequence of any 0+ chars other than linebreak symbols as many as possible up to the last !. sequence and then matches {display:none}. Since your text does not contain ! nor the next . you have no match.
One way to get your matches is to use \w+ (=one or more word characters, those in the [a-zA-Z0-9_] set) pattern inside Group 1:
\.(\w+){display:none}
See the regex demo
preg_match_all('@\.(\w+){display:none}@', '.UlEa{display:none}.RNLW{display:inline}.jc0k{display:none}.Lyhf{display:inline}', $matches);
print_r($matches[1]); // => Array ( [0] => UlEa [1] => jc0k )