PHP，preg_replace和正则表达式的问题

I'm trying to run this php command:

preg_replace($regexp, $replace, $text, $maxsingle);

Where the vars are:

$regexp = '/(?!(?:[^<\\[]+[>\\]]|[^>\\]]+<\\/a>))\\b(שלום)\\b/imsU';
$replace = '<a title="$1" href="http://stackoverflow.com">$1</a>';
$text is a long post
$maxsingle = 3;

When the text I'm trying to match (in the above case "שלום") is in english everything works. However, when the text is Hebrew, it doesn't matches anything...

Any ideas how to make Hebrew work with preg_replace?

Thanks.

Try using the /u (utf-8) flag