$re = '/^StartWord\R+\K.*?\R(?=\R*^StopWord\R)/ms';
$str = 'Delete this text De l e te this text
StartWord
Apples Oranges
Pears Grapes
StopWord
Delete this text Delete this text
StartWord
Peas Carrots
Peas Carrots
StopWord
Delete this text Delete this text
';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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