$re = '/Start((?!Start).)*email1\@gmail\.com(.*?)End([^\n]*)/s';
$str = 'Start of a transaction.
random line 1.
random line 2.
email1@yahoo.com # WRONG MAIL!
random line 3.
End of a transaction.
Start of a transaction.
random line 1.
random line 2.
email1@gmail.com
End of a transaction.
Start of a transaction.
random line 1.
random line 2.
email1@yahoo.com
random line 3.
End of a transaction.
Start of a transaction.
random line 1.
random line 2.
email1@gmail.com
End of a transaction.';
preg_match_all($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