Description:
------------
It's not possible to use the $n style backreference in the pattern of the PCRE functions.

A pattern like this:

/\[(u|b|i|s)\](.*)\[\/$1\]/si

Should match [b]bold[/b]. However, it does not.
This pattern though:

/\[(u|b|i|s)\](.*)\[\/\\1\]/si

_Will_ match [b]bold[/b].

Reproduce code:
---------------
<?php

function parseUBB($sString)
{
	
	// Bold, underline, italic and strike tags
	$sString = preg_replace("/\[(u|b|i|s)\](.*)\[\/$1\]/si", "<$1>$2</$1>", $sString);
	
	return ($sString);
}

echo parseUBB("[b]testb[/b], [u]testu[/u], [i]testi[/i]");

?>

Expected result:
----------------
<b>testb</b>, <u>testu</u>, <i>testi</i>

Actual result:
--------------
[b]test[/b], [u]testu[/u], [i]testi[/i]