Description:
------------
While converting phpBB code to html, preg_replace return empty string.

Test case:
<?php
	$str = '[b]';
	// Please try run this test case also with $i<99997 and compare result's
	for ($i=0; $i<99998; $i++) $str .= 'q';
	$str = preg_replace("#\[b\](.+?)\[\/b\]#is", "[p]", $str);
	echo $str;
?>

I know about pcre.backtrack_limit, but i think in this case PCRE must generate 
exception with fatal error.

Test script:
---------------
<?php
	$str = '[b]';
	// Please try run this test case also with $i<99997 and compare result's
	for ($i=0; $i<99998; $i++) $str .= 'q';
	$str = preg_replace("#\[b\](.+?)\[\/b\]#is", "[p]", $str);
	echo $str;
?>

Expected result:
----------------
Return non empty string or generate exception with fatal error

Actual result:
--------------
Empty string