_Agh! I fat fingered submitting my comment before I was done. ;)_

It is not legal to end a version with a `+`.

So I tend to think that `Identifiers MUST NOT be empty` is redundant in light of ` Identifiers MUST comprise only ASCII alphanumerics and hyphen [0-9A-Za-z-]` After all, the empty character violates that regex.

But I can see how the `only` introduces ambiguity. Only could be interpreted to mean "if there are any characters, they must be in this set." I don't think many folks are confused into thinking that empty identifiers are allowed. After all, it wouldn't make much sense and none of the examples show it.

I wonder if we can get away with an even shorter clarification.

`Identifiers MUST comprise one or more ASCII alphanumerics and hyphen [0-9A-Za-z-]`

Alternatively, we could clarify it earlier.

`A pre-release version MAY be denoted by appending a hyphen and a series of non-empty dot separated identifiers`

Thoughts?