How does Content Object -> match Method work properly

In the zimlet definition file there is an easy hook to put a regex match to an email body:

<matchOn>
<regex attrs="ig">lab|user|remodel</regex> <!-- matches lab or user or remodel -->
</matchOn>

Two questions, whenever a mail is opened the .match event fires like 30 times.

Is there an event that fires only once? If not what is the proper attribute to check if there was a match AND how to get the matched content.

I cannot find it in the documentation, or examples.

Thanks, Barry!!