I was getting quite confused by

 darcs diff --from-tag foo-0.1 --to-tag foo-0.2

showing changes after tag foo-0.2, because of a later tag foo-0.2.1.

It seems preferring exact tag matches would be less surprising behaviour.

A workaround is "darcs diff --from-tag=foo-0.1$ --to-tag=foo-0.2$".

Thinking about this a bit more, it seems like we should keep things as
they are because

* matching regexes on tag names allows for idioms like "darcs changes
--from-tag ."
* it does not guarantee uniqueness, because as you observed on IRC, tags
do not have to have unique names (imagine the case of merging two
repos)... but it would *appear* to be unique *most* of the time, which
could mislead people
* keeping the behaviour as uniform and exception-free as possible keeps
Darcs friendly and learnable
* it also lets us say things like "a tag is just a patch with a special
name"

If you feel this needs discussion, please open a thread on darcs-users
(that's where we do all our thinking; the BTS is just a place to note
down where things are)

Thanks for the report in the meantime and hope discovering Darcs is a
pleasant experience. :-)