Unit test - more strict format string

Changes:

* hex digit character class
* more strict rfc4122 compliance for variant (see [Wikipedia](http://en.wikipedia.org/wiki/Universally_unique_identifier#Variants_and_versions)):

> The variant covered by the UUID specification is indicated by the two most significant bits of N being 1 0 (i.e., the hexadecimal N will always be 8, 9, A, or B).

This depends on #6 to be merged in order to pass.