wpautop regular expression issue (default filter)

I’m trying to import movable type format data into wordpress. Wpautop
default filter contains

$pee = preg_replace(‘/\n?(.+?)(?:\n\s*\n|\z)/s’, “<p>$1</p>\n”, $pee); // make paragraphs, including one at the end.

But it’s not work for few pages and when I add U (PCRE_UNGREEDY) to above modifiers it’s work.
$pee = preg_replace(‘/\n?(.+?)(?:\n\s*\n|\z)/sU‘, “<p>$1</p>\n”, $pee);

Actually I want to work above reg exp without modifying WP code.[adding U (PCRE_UNGREEDY) modifier]

Will you please help to achive above task.