$re = '/(?>\p{Lu}+?\s?)+\(?\p{Lu}+\)?\K(:)|(?<!\w)(\s)(?=\p{Lu}{2,})/m';
$str = 'this is a speech text. FIRST PERSON: hi all, thank you for coming. SECOND PERSON: thank you for inviting us. TERCER PERSONA QUE SE LLAMA ANDRÉS: soy de: acuerdo. CUARTA PERSONA (JOHN): Hi. How are you? ANDRÉS: Hola buenos días!';
preg_match_all($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