$re = '/Header[^:]+:([\s\S]*?)(?=Header|$)/';
$str = 'Header One: Some random text Header Two: Some more text Header One: Some random textwerwerwefvxcf234234 Header Three: Some more t2345fsdfext Header Four: Some randsdfsdf3w42343om text Header Five: Some more text 123213';
preg_match_all($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