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

[Ausum]

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