formatting.php:shortcode_unautop() empties the post

WordPress 3.1-RC4 with Lazyest gallery 1.1.beta-3. Other plugins are turned off. Not sure if it is a regexp or preg_replace problem.

Depending on one letter presence the function shortcode_unautop() empties whole post. I’ve prepared a small testcase with part of shortcode_unautop() code to reproduce this problem:

See file https://www.andrianov.ru/wp-shortcode.tar.gz, there are two files. $pee variable has the same value as $pee in shortcode_unautop(). Other variables are derived from corresponding variables in shortcode_unautop(). Note, the difference between 2 posts is only one letter.

$ php good.php
Len before: 18876
Len after: 18870
$ php bad.php
Len before: 18877
Len after: 1
$ diff good.php bad.php
43c43
< AAAA</p>
—
> AAAAA</p>