grep doesn't handle \w correctly

Put the following text into a file called testfile.txt:

-------testfile.txt-------
if ($this->_touchOnly) //log if touchOnly
only log the output of the controller if $silent
if (window.console) console.log(data);

ERunActions::runAction($logOutput=false,$silent=false)
self::logTrace('Action output: ' . $output,$controller->id,$actionId);
self::logError($msg,$controller->id,$actionId);
-------end of testfile.txt

Try this command:
$ grep -Re "[^\w]log[^\w]" testfile.txt

Expected result:
Should find matches only in lines 1,2,3, gighlighting the word "log"

Observed result:
Finds also matches in lines 5,6,7, highlighting the strings "logO", "logT" and "logE" respectively.

According to the man page:
    The symbol \w is a synonym for [_[:alnum:]]

And in turn:
    For example, [[:alnum:]] means the character class of numbers and
       letters in the current locale. In the C locale and ASCII character set
       encoding, this is the same as [0-9A-Za-z].

It looks like, instead, \w is interpreted as only numbers and LOWERCASE letters, but not uppercase letters.

I know it seems unbelievable to be finding a bug in grep in 2013, but compare to
http://regexr.com?34e4i
which shows the expected result

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: grep 2.12-2
ProcVersionSignature: Ubuntu 3.5.0-27.46-generic 3.5.7.7
Uname: Linux 3.5.0-27-generic i686
NonfreeKernelModules: nvidia
ApportVersion: 2.6.1-0ubuntu10
Architecture: i386
Date: Sat Apr 6 22:27:39 2013
InstallationDate: Installed on 2010-06-23 (1018 days ago)
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release i386 (20100429)
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: grep
UpgradeStatus: Upgraded to quantal on 2013-01-13 (83 days ago)