Generating all instances of a regular language up to a certain length
There are several recent papers that address your question, e.g.:

Margareta Ackerman and Jeffrey Shallit. Efficient Enumeration of Regular Languages. Conference on Implementation and Application of Automata (CIAA) , Lecture Notes in Computer Science, 4783, Springer-Verlag, Berlin Heidelberg, pp. 226-241, 2007.

Margareta Ackerman and Erkki Makinen. Three New Algorithms for Regular Language Enumeration, Computing and Combinatorics: 15th Annual International Conference (COCOON) , Lecture Notes in Computer Science 5609, Springer-Verlag, Berlin Heidelberg, pp. 178-191, 2009.

You can get these two papers from here.