How to write regex to ignore?

Hi!

I don't get the "Ignore regexp" for html-tags to work. I want to exclude html comment tags (<!--like this one-->) and tried with something like:

\<\!\-\-(.*?)\-\-\>

but then I get the error message:
"warning: preg_replace_callback() [function.preg-replace-callback]: Delimiter must not be alphanumeric or backslash in .../sites/all/modules/pearwiki_filter/Text/Wiki/Parse.php on line 156."

And, if I try

<!--(.*?)-->

It doesn't leave the comment alone (as code), but shows it as text.

If anyone could just show an example of a regex that works for them, that would be a great help.