[Zope] How to filter unwanted strings within a form's textarea?

I need my local users to create content using html tags within a textarea, but I don't want them to include links to external images resources. My idea is to enable a filter that looks for the "http://www.whatever_ext_site.com" string within the img tag, and then erase it before posting the whole form data to the proper method. Is there a simple way for doing this? (If not, what is the hard way? :) ) Thanks in advance, Ausum