$re = '/(?:\G(?!\A)|\[)[a-zA-Z.]*\K(?:(А)|(а)|(І)|(і)|(В)|(С)|(с)|(Е)|(е)|(Р)|(Т)|(т)|(О)|(о)|(К)|(к)|(Х)|(х)|(М))(?=[[:alpha:].]*])/mu';
$str = '[Root.GetSomething] [Rооt.GеtSоmеthіng]';
preg_match_all($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