Regular Expressions are usually expressed as strings, but they also have properties (ie. single line, multi line, ignore case). How would you store them? And for compiled regular expressions, how to store it?
Please note that we can write custom property classes: http://googleappengine.blogspot.com/2009/07/writing-custom-property-classes.html
As I don't understand Python enough, my first try to write a custom property which stores a compiled regular expression failed.
You can either store the text, as suggested above, or you can pickle and unpickle the compiled RE. For example, see PickledProperty on the cookbook.
Due to the (lack of) speed of Pickle, particularly on App Engine where cPickle is unavailable, you'll probably find that storing the text of the regex is the faster option. In fact, it appears that when pickled, a re simply stores the original text anyway.