$re = '/(?(DEFINE)
(?<u>[А-ЯЁ])
(?<l>[а-яё])
(?<b>
(?<=^)(?=[а-яёА-ЯЁ])
|
(?<=[а-яёА-ЯЁ])(?=$)
|
(?<=[^а-яёА-ЯЁ])(?=[а-яёА-ЯЁ])
|
(?<=[а-яёА-ЯЁ])(?=[^а-яёА-ЯЁ])
)
)
(?P>b)
(?:
(?P>l)+(?P>u)(?P>l)+
|
(?P>u)(?P>l)+
|
(?P>l)+(?P>u)
)
(?P>b)/umx';
$str = 'Русское слово имЕющее роВНо ОДНу заглавнуЮ';
preg_match_all($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