Apple has (or had, I'm not sure what the status is) a technology called Data
Detectors.  Data Detectors allowed you to highlight some text in any Mac
application and pop up a contextual menu which, based on the kinds of
DataDetector modules you had installed, would parse the selected text and
look for things like URLs, email addresses, currencies, etc.  

If DataDetectors found a URL in the selected text, for example, the pop-up
menu would have one or more options to open that URL in any of the browsers
that you had installed on your computer.  One of the neat things about this
was that you didn't have to select the URL exactly, you could select a block
of text with the URL embedded somewhere within.

The DataDetector class does all the work of figuring out what can be done
with the selected text.

Obviously this is useful mostly from a GUI perspective.  But the concept
here is to assign responsibility for behavior to the appropriate class.

Rob