Description:
------------
PHP: 5.2.14, Apache 2.2.15, mod_php

While \s is supposed to match all whitespace, the greek unicode letter Pi (Π) whose code is 0xCEA0 is matched too and if replaced with something, it's stripped of its second byte (0xA0).

Test script:
---------------
<?php
mb_internal_encoding('UTF-8');

$testStr = 'Π  Π  Π!';
$newStr = mb_ereg_replace('\s+','_',$testStr);
echo $testStr;
echo $newStr;
echo urlencode($testStr);
echo urlencode($newStr);
?>

Expected result:
----------------
Π  Π  Π!
Π__Π__Π!
%CE%A0++%CE%A0++%CE%A0%21
%CE%A0__%CE%A0__%CE%A0%21

Actual result:
--------------
Π  Π  Π!
[non printable character]_[non printable character]_[non printable character]!
%CE%A0++%CE%A0++%CE%A0%21
%CE_%CE_%CE_%21